selftests/kselftest/runner.sh: Add 45 second timeout per test

Commit a745f7af ("selftests/harness: Add 30 second timeout per
test") solves the problem of kselftest_harness.h-using binary tests
possibly hanging forever. However, scripts and other binaries can still
hang forever. This adds a global timeout to each test script run.

To make this configurable (e.g. as needed in the "rtc" test case),
include a new per-test-directory "settings" file (similar to "config")
that can contain kselftest-specific settings. The first recognized field
is "timeout".

Additionally, this splits the reporting for timeouts into a specific
"TIMEOUT" not-ok (and adds exit code reporting in the remaining case).

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>