Optimize pipeline for cases where only a test file is being created

In this MR !17610 (merged) only a new test was created (only a new test file), but the whole Test job was run, when we could be running simply the new test, which would provide much faster feedback to the MR's author.

So, the idea is that we should have an optimized pipeline for such cases.

Cc: @kwiebers, @rymai, and @zeffmorgan.