Documentation: KUnit: make usage.rst a superset of tips.rst, remove duplication

usage.rst had most of the content of the tips.rst page copied over.
But it's missing https://www.kernel.org/doc/html/v6.0/dev-tools/kunit/tips.html#customizing-error-messages


Copy it over so we can retire tips.rst w/o losing content.

And in that process, it also gained a duplicate section about how
KUNIT_ASSERT_*() exit the test case early. Remove that.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Sadiya Kazi <sadiyakazi@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>