Require bundler gem from spec_helper

What does this MR do?

Require the bundler gem from spec/spec_helper.rb

Why was this MR needed?

The tests failed with one of these two errors:

     NameError:
       uninitialized constant Bundler

or

     NameError:
       uninitialized constant Helpers::Bundler

This is the same really.

See https://gitlab.com/gitlab-org/security-products/dependency-scanning/-/jobs/73077058

This regression occurred when updating to the latest version of Ruby 2.3. Same issue with 2.4. Tests run fine with 2.2.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug

What are the relevant issue numbers?

None.