selftests: sync: kselftest and kselftest-clean fail for make O=dir case

sync test fails to build when object directory is specified to relocate
object files. Fix it to specify the correct path. Fix clean target to
remove objects. Also include simplified logic to use TEST_CUSTOM_PROGS
in build and clean targets instead of hard-coding the test name each
time.

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