kunit: Warn if tests are slow

Kunit recently gained support to setup attributes, the first one being
the speed of a given test, then allowing to filter out slow tests.

A slow test is defined in the documentation as taking more than one
second. There's an another speed attribute called "super slow" but whose
definition is less clear.

Add support to the test runner to check the test execution time, and
report tests that should be marked as slow but aren't.

Signed-off-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>