selftests: check before install

When the test cases is not supported by the current architecture
the install files(TEST_PROGS, TEST_PROGS_EXTENDED and TEST_FILES)
will be empty. Check it before installation to dismiss a failure
reported by install program.

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>