Improve spec/lib/gitlab/git_access_snippet_spec spec

What does this MR do?

In this MR we're improving spec/lib/gitlab/git_access_snippet_spec.rb spec.

	Time	Queries
Before the MR	28.3s	5342
After the MR	24.3s	3335

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #228704 (closed)