kunit: rename print_subtest_{start,end} for clarity (s/subtest/suite)

These names sound more general than they are.

The _end() function increments a `static int kunit_suite_counter`, so it
can only safely be called on suites, aka top-level subtests.
It would need to have a separate counter for each level of subtest to be
generic enough.

So rename it to make it clear it's only appropriate for suites.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>