Rename SOURCE_STEP to EXTRACT_STEP

[Flamefire]

Adjust tests to take that into account and add tests to check that the old name causes a proper error. Add such error handling for the skipsteps EC parameter.

Split up test_skip into more focused sub tests to allow adding the new (negative/failure) test of skipsteps to one of those.

Also a fix for test_compiler_cache and a bit cleanup to avoid copying the same superflous parameters in the new tests.

Fixes #4627