kunit: time: Mark test as slow using test attributes

Mark the time KUnit test, time64_to_tm_test_date_range, as slow using test
attributes.

This test ran relatively much slower than most other KUnit tests.

By marking this test as slow, the test can now be filtered using the KUnit
test attribute filtering feature. Example: --filter "speed>slow". This will
run only the tests that have speeds faster than slow. The slow attribute
will also be outputted in KTAP.

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