kunit: drop unused kunit* field in kunit_assert

The `struct kunit* test` field in kunit_assert is unused.
Note: string_stream needs it, but it has its own `test` field. I assume
`test` in `kunit_assert` predates this and was leftover after some
refactoring.

This patch removes the field and cleans up the macros to avoid
needlessly passing around `test`.

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