Improve comments for @pytest.mark.parametrize test cases in `test_scale_to` function

[bobleesj]

          @sbillinge ready for review.

@yucongalicechen , the part that is remaining is the test_scale_to function.

The existing comments like# C2: Same length x-arrays with exact x-value match, # C3: Same length x-arrays with approximate x-value match were abit hard for me to understand what's being tested. Would you mind improving this test comments for test_scale_to using the above format?

Originally posted by @bobleesj in #273 (comment)

[yucongalicechen]

I can take this issue!