selftests: media_tests: Fix Makefile 'clean' target warning

Remove 'clean' target and change TEST_PROGS to TEST_GEN_PROGS so the
common lib.mk 'clean' target clean these generated files. TEST_PROGS
is for shell scripts and not for generated test executables.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>