Fix gitaly rspec failures 'too many open files'

What does this MR do?

The new cat-file cache in Gitaly will keep up to 100 git cat-file processes around for re-use. This default is not a good number for development on macOS, where you will run against the "open files" resource limit (ulimit -n) way before you can spawn 100 processes. This meant that something like rspec spec/lib/gitlab/git on macOS would start failing half-way through.

This MR changes the configuration of gitaly in rspec so that it keeps no more than 5 cat-file processes in its cache.

Part of gitaly#1677 (closed)

Fixes gitaly#1711 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
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• Style guides
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• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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