Improve regex used in "require_migration!" test helper

What does this MR do?

I've noticed this while working on !64159 (merged).

Vim (and probably other editors) create hidden temp files for the files that are open for editing.

Example: If we open migration file like

20210615234935_fix_batched_migrations_old_format_job_arguments.rb

Vim will create file named

.20210615234935_fix_batched_migrations_old_format_job_arguments.rb.swp

When running the migrations test spec/migrations/fix_batched_migrations_old_format_job_arguments_spec.rb, require_migration! test helper will fail with

LoadError:
  cannot load such file -- /Users/krasio/projects/gitlab/ee/gitlab/db/post_migrate/.20210615234935_fix_batched_migrations_old_format_job_arguments.rb.swp
# ./spec/support/helpers/require_migration.rb:25:in `require_migration!'
# ./spec/support/helpers/require_migration.rb:51:in `require_migration!'
# ./spec/migrations/fix_batched_migrations_old_format_job_arguments_spec.rb:4:in `<top (required)>'

because it pick up the swp file, instead of the migration file.

This MR updates the helper to match only on the file's basename, and also updates the regex used to find the migration file, so that it does not match swp files.

• The regex before - https://rubular.com/r/J1pTK1abHAJjxi.
• The regex after - https://rubular.com/r/NZhPDcbzBwtJYw.

Does this MR meet the acceptance criteria?

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