Add test for abstract shell guardGetSourcesScriptHooks method
What does this MR do?
Add test for abstract shell guardGetSourcesScriptHooks method.
Before starting work on #38703 (closed), I added the missing tests.
Why was this MR needed?
For the testability of changes.
What's the best way to test this MR?
go test -timeout 30s -run ^TestAbstractShell_guardGetSourcesScriptHooks$ gitlab.com/gitlab-org/gitlab-runner/shells