Add spec to verify import test json

We want to test Import on "complete" data set,
which means that every relation (as in our Import/Export definition)
is covered.
At least one of the test JSONs, such as
`spec/fixtures/lib/gitlab/import_export/complex/project.json`
should include them (and being a non-empty relation).

spec/lib/gitlab/import_export/import_test_coverage_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+# We want to test Import on "complete" data set,
+# which means that every relation (as in our Import/Export definition) is covered.
+# Fixture JSONs we use for testing Import such as
+# `spec/fixtures/lib/gitlab/import_export/complex/project.json`
+# should include these relations being non-empty.
+describe 'Test coverage of the Project Import' do
+  include ConfigurationHelper
+
+  # `MUTED_RELATIONS` is a technical debt.
+  # This list expected to be empty or used as a workround
+  # in case this spec blocks an important urgent MR.
+  # It is also expected that adding a relation in the list should lead to
+  # opening a follow-up issue to fix this.
+  MUTED_RELATIONS = %w[
+    project.milestones.events.push_event_payload
+    project.issues.events.push_event_payload
+    project.issues.notes.events
+    project.issues.notes.events.push_event_payload
+    project.issues.milestone.events.push_event_payload
+    project.issues.issue_milestones
+    project.issues.issue_milestones.milestone
+    project.issues.resource_label_events.label.priorities
+    project.issues.designs.notes
+    project.issues.designs.notes.author
+    project.issues.designs.notes.events
+    project.issues.designs.notes.events.push_event_payload
+    project.merge_requests.metrics
+    project.merge_requests.notes.events.push_event_payload
+    project.merge_requests.events.push_event_payload
+    project.merge_requests.timelogs
+    project.merge_requests.label_links
+    project.merge_requests.label_links.label
+    project.merge_requests.label_links.label.priorities
+    project.merge_requests.milestone
+    project.merge_requests.milestone.events
+    project.merge_requests.milestone.events.push_event_payload
+    project.merge_requests.merge_request_milestones
+    project.merge_requests.merge_request_milestones.milestone
+    project.merge_requests.resource_label_events.label
+    project.merge_requests.resource_label_events.label.priorities
+    project.ci_pipelines.notes.events
+    project.ci_pipelines.notes.events.push_event_payload
+    project.protected_branches.unprotect_access_levels
+    project.prometheus_metrics
+    project.metrics_setting
+    project.boards.lists.label.priorities
+    project.service_desk_setting
+  ].freeze
+
+  # A list of JSON fixture files we use to test Import.
+  # Note that we use separate fixture to test ee-only features.
+  # Most of the relations are present in `complex/project.json`
+  # which is our main fixture.
+  PROJECT_JSON_FIXTURES = [
+    'spec/fixtures/lib/gitlab/import_export/complex/project.json',
+    'spec/fixtures/lib/gitlab/import_export/group/project.json',
+    'spec/fixtures/lib/gitlab/import_export/light/project.json',
+    'spec/fixtures/lib/gitlab/import_export/milestone-iid/project.json',
+    'ee/spec/fixtures/lib/gitlab/import_export/designs/project.json'
+  ].freeze
+
+  it 'ensures that all imported/exported relations are present in test JSONs' do
+    not_tested_relations = (relations_from_config - tested_relations) - MUTED_RELATIONS
+
+    expect(not_tested_relations).to be_empty, failure_message(not_tested_relations)
+  end
+
+  def relations_from_config
+    relation_paths_for(:project)
+      .map { |relation_names| relation_names.join(".") }
+      .to_set
+  end
+
+  def tested_relations
+    PROJECT_JSON_FIXTURES.flat_map(&method(:relations_from_json)).to_set
+  end
+
+  def relations_from_json(json_file)
+    json = ActiveSupport::JSON.decode(IO.read(json_file))
+
+    Gitlab::ImportExport::RelationRenameService.rename(json)
+
+    [].tap {|res| gather_relations({ project: json }, res, [])}
+      .map {|relation_names| relation_names.join('.')}
+  end
+
+  def gather_relations(item, res, path)
+    case item
+    when Hash
+      item.each do |k, v|
+        if (v.is_a?(Array) || v.is_a?(Hash)) && v.present?
+          new_path = path + [k]
+          res << new_path
+          gather_relations(v, res, new_path)
+        end
+      end
+    when Array
+      item.each {|i| gather_relations(i, res, path)}
+    end
+  end
+
+  def failure_message(not_tested_relations)
+    <<~MSG
+      These relations seem to be added recenty and
+      they expected to be covered in our Import specs: #{not_tested_relations}.
+
+      To do that, expand one of the files listed in `PROJECT_JSON_FIXTURES`
+      (or expand the list if you consider adding a new fixture file).
+
+      After that, add a new spec into
+      `spec/lib/gitlab/import_export/project_tree_restorer_spec.rb`
+      to check that the relation is being imported correctly.
+
+      In case the spec breaks the master or there is a sense of urgency,
+      you could include the relations into the `MUTED_RELATIONS` list.
+
+      Muting relations is considered to be a temporary solution, so please
+      open a follow-up issue and try to fix that when it is possible.
+    MSG
+  end
+end