Fix date-dependent test failure in obsolete_ignored_columns_spec.rb

If `spec/lib/gitlab/database/obsolete_ignored_columns_spec.rb` is loaded
before a date boundary and execute after the boundary has crossed, the
test will fail with an extra column because `Date.today` has advanced.
To fix this problem, we fix the dates used in the test.

spec/lib/gitlab/database/obsolete_ignored_columns_spec.rb

@@ -4,6 +4,9 @@ require 'spec_helper'
 
 describe Gitlab::Database::ObsoleteIgnoredColumns do
   module Testing
+    # Used a fixed date to prevent tests failing across date boundaries
+    REMOVE_DATE = Date.new(2019, 12, 16)
+
     class MyBase < ApplicationRecord
     end
 
@@ -23,12 +26,12 @@ describe Gitlab::Database::ObsoleteIgnoredColumns do
       self.table_name = 'issues'
 
       ignore_column :id, :other, remove_after: '2019-01-01', remove_with: '12.0'
-      ignore_column :not_used_but_still_ignored, remove_after: Date.today.to_s, remove_with: '12.1'
+      ignore_column :not_used_but_still_ignored, remove_after: REMOVE_DATE.to_s, remove_with: '12.1'
     end
 
     class A < SomeAbstract
       ignore_column :also_unused, remove_after: '2019-02-01', remove_with: '12.1'
-      ignore_column :not_used_but_still_ignored, remove_after: Date.today.to_s, remove_with: '12.1'
+      ignore_column :not_used_but_still_ignored, remove_after: REMOVE_DATE.to_s, remove_with: '12.1'
     end
 
     class C < MyBase
@@ -40,6 +43,7 @@ describe Gitlab::Database::ObsoleteIgnoredColumns do
 
   describe '#execute' do
     it 'returns a list of class names and columns pairs' do
+      Timecop.freeze(Testing::REMOVE_DATE) do
         expect(subject.execute).to eq([
           ['Testing::A', {
             'unused' => IgnorableColumns::ColumnIgnore.new(Date.parse('2019-01-01'), '12.0'),
@@ -51,4 +55,5 @@ describe Gitlab::Database::ObsoleteIgnoredColumns do
         ])
       end
     end
+  end
 end