1133 - more reorg of commandline parsing

1. Drop the ability to run just some C++ tests. It's a lousy interface
to use line numbers, we can't selectively run mu tests, we haven't used
it in a while because our tests run plenty fast, and it's complicating
other parts, like Next_recipe_number and test space handling.

2. Create a new layer right up top to show the usage message and all the
different forms that are possible. Good for documentation until we come
up with a way to put the different forms in their own layers. At least
it's out of the test layer now.

3. Strengthen the assertion that no recipes exist in test space before
we start running any sorts of tests. Earlier it was possible for
files loaded explicitly to overflow into test space because we were
asserting before load, not after. Now we check if we need to run tests,
load all files, then make the assertion, run tests, and exit if
necessary.

Now we don't need to mess with commandline args at all in layer 50.
That's a sign that we're on the right track.

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