Add healthcheck polling for mock server container
What does this MR do and why?
Adds a healthcheck for mock server startup and uses built in retry if mock server is unhealthy.
This is related to occasional test failures where mock server would be started but would respond with 500 status code to all requests due to some internal ssl issue.
Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/412601
MR acceptance checklist
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