selftests/harness: Allow test to configure timeout

Commit a745f7af ("selftests/harness: Add 30 second timeout per test")
adds an hardcoded 30s timeout to all tests. Unfortunately, rtctest has two
tests taking up to 60s. Allow for individual tests to define their own
timeout.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>