Makefile: Add run-test.mk to clean

[KrystalDelusion]

What are the reasons/motivation for this change?

If a test directory already contains a run-test.mk file it won't regenerate if anything in the directory changes, meaning any files will be skipped and any removed tests will raise an error because of the missing file.

Explain how this is achieved.

MK_TEST_DIRS provides a list of directories that generate run-test.mk files, so we can use that list in the clean target to delete them; thus allowing future runs of make test to correctly regenerate the needed file.

If applicable, please suggest to reviewers how they can test the change.

make tests/techmap/run-test.mk
rm tests/techmap/abc9.ys
clean
make makefile-tests/tests/techmap

Without this change, the second call to make will raise an error due to the missing abc9.ys.