selftests: kselftest_deps: fix l5_test() empty variable

In the function l5_test(), variable $tests is empty when there is no .mk
file in the subsystem to be tested. It causes the following grep operation
get stuck.

This fix check the variable $tests, return when it is empty.

Signed-off-by: Lu Dai <dai.lu@exordes.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>