test: Skip cleanup test if not built out of tree

42c0e5bb ("test: Find leftovers after clean/mrproper") assumes out
of tree builds. test_clean and test_mrproper fail if the assumption
isn't met.

However there are valid scenarios where tests run with in source builds
(i.e., test.py --build-dir=.).

Automatically skip cleanup tests in such scenarios to prevent false
positives.

Signed-off-by: Tobias Deiminger <tdmg@linutronix.de>
Reviewed-by: Harald Seiler <u-boot@rahix.de>