Fix broken handling for outdated test repos

What does this MR do?

Fixes the error handling when one of the test repositories in tmp/tests is outdated, see e.g. the recent problems with the commit 99e4853 in Slack.

The call to TestEnv.init had the side effect of starting another Gitaly process, and didn't actually correctly delete the test repositories (unless vs. if)

Not sure about the comment referencing --mirror which we don't seem to use, but I'm assuming a fetch will usually be fine and if not the developers can keep deleting tmp/tests as they are doing now :)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
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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
• Tested manually

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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