Unclear testing directions

Looking at docs/TESTS.md, the editing section is unclear. What is `--`? I know we're essentially commenting out the test, but the student may not catch that. Also, what happens if a test is across multiple lines? There also aren't instructions on how specifically to enable the tests. It's implied we're supposed to uncomment the test, but a student may miss this piece of information.

We're also telling students to enable tests until they're passing, but we're not indicating how to know if a test is passing.

Read docs/TESTS.md, focusing on the editing section. Clarify what `--` means, how to handle tests spanning multiple lines, how to enable tests, and how students can tell when a test is passing; done when the instructions answer each of these questions clearly.

Written by the indexing model from the issue text.