Fix order-dependent migration specs

CleanupOptimisticLockingNulls and CleanupOptimisticLockingNullsPt2 both
defined the constant TABLES, which caused the tests to fail if they were
run together. To fix this, we drop the constant and use straight
definitions.

spec/migrations/cleanup_optimistic_locking_nulls_pt2_spec.rb

@@ -4,11 +4,10 @@ require 'spec_helper'
 require Rails.root.join('db', 'post_migrate', '20200217210353_cleanup_optimistic_locking_nulls_pt2')
 
 describe CleanupOptimisticLockingNullsPt2, :migration do
-  TABLES = %w(ci_stages ci_builds ci_pipelines).freeze
-  TABLES.each do |table|
-    let(table.to_sym) { table(table.to_sym) }
-  end
-  let(:tables) { TABLES.map { |t| method(t.to_sym).call } }
+  let(:ci_stages) { table(:ci_stages) }
+  let(:ci_builds) { table(:ci_builds) }
+  let(:ci_pipelines) { table(:ci_pipelines) }
+  let(:tables) { [ci_stages, ci_builds, ci_pipelines] }
 
   before do
     # Create necessary rows

spec/migrations/cleanup_optimistic_locking_nulls_spec.rb

@@ -4,11 +4,10 @@ require 'spec_helper'
 require Rails.root.join('db', 'post_migrate', '20200128210353_cleanup_optimistic_locking_nulls')
 
 describe CleanupOptimisticLockingNulls do
-  TABLES = %w(epics merge_requests issues).freeze
-  TABLES.each do |table|
-    let(table.to_sym) { table(table.to_sym) }
-  end
-  let(:tables) { TABLES.map { |t| method(t.to_sym).call } }
+  let(:epics) { table(:epics) }
+  let(:merge_requests) { table(:merge_requests) }
+  let(:issues) { table(:issues) }
+  let(:tables) { [epics, merge_requests, issues] }
 
   let(:namespaces) { table(:namespaces) }
   let(:projects) { table(:projects) }