test: Find leftovers after clean/mrproper

Docs describe 'make clean' to delete most generated files, 'make
mrproper' to delete current configuration and all generated files. This
test tries to assert it.

Idea is to search remaining files by patterns in copies of the initial
out-of-source build, which has two advantages:
- looking in an out-of-source build dir allows to tell generated source
  code from committed source code
- copying is fast (compared to rebuilding each time) which allows to do
  a "world clean"

Signed-off-by: Tobias Deiminger <tdmg@linutronix.de>