We had some simple docs on how to run our unit test suites, covering
subsets of tests and briefly talking about pdb, but it was buried in the
Getting Started guide and didn't cover a number of the more useful
tricks our test suites allow for.

This introduces a more prominent, complete set of documentation on our
test suites, covering how to run subsets of tests, working with failed
tests, debugging test failures, showing coverage reports, and profiling
tested code.

The old docs now point to this new guide, to help transition people.

Generated and read through the new docs. Didn't see any doc errors.
Ran unit tests.
Tested all the links.