Remove config.Config from server package
Usage of the global var config.Config replaced with
the local setup of the configuration.
Gitaly-ruby sidecar initialisation removed as it is not
used and it speeds up test execution for a bit.
Test code refactored to replace t.Fatal with require.NoError, defer cleanup()
with t.Cleanup(cleanup)
and testhelpers marked with t.Helper()
call.
Part of: #2699 (closed)