kunit: Adjust kunit_test timeout based on test_{suite,case} speed

Currently, the in-kernel kunit test case timeout is 300 seconds. (There
is a separate timeout mechanism for the whole test execution in
kunit.py, but that's unrelated.) However, tests marked 'slow' or 'very
slow' may timeout, particularly on slower machines.

Implement a multiplier to the test-case timeout, so that slower tests
have longer to complete:
- DEFAULT -> 1x default timeout
- KUNIT_SPEED_SLOW -> 3x default timeout
- KUNIT_SPEED_VERY_SLOW -> 12x default timeout

A further change is planned to allow user configuration of the
default/base timeout to allow people with faster or slower machines to
adjust these to their use-cases.

Link: https://lore.kernel.org/r/20250614084711.2654593-2-davidgow@google.com


Signed-off-by: Ujwal Jain <ujwaljain@google.com>
Co-developed-by: David Gow <davidgow@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>