Promote Timecop gem into joint test/development Bundler group

Timecop was being used in the cycle analytics and burndown seed_fu tasks.
While it seems a little sketchy to use Timecop in this way, this seems like
an exception can be made for test fixtures.

Closes #44751, gitlab-org/gitlab-ee#5359

Gemfile

@@ -376,6 +376,8 @@ group :development, :test do
   gem 'stackprof', '~> 0.2.10', require: false
 
   gem 'simple_po_parser', '~> 1.1.2', require: false
+
+  gem 'timecop', '~> 0.8.0'
 end
 
 group :test do
@@ -385,7 +387,6 @@ group :test do
   gem 'webmock', '~> 2.3.2'
   gem 'test_after_commit', '~> 1.1'
   gem 'sham_rack', '~> 1.3.6'
-  gem 'timecop', '~> 0.8.0'
   gem 'concurrent-ruby', '~> 1.0.5'
   gem 'test-prof', '~> 0.2.5'
 end