selftests/zram: Makefile fix

Do not override run_tests, The default rule will just run TEST_PROGS

Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>