[mypyc] Improve access to generated C on test failures and document this (#18476)

Now the generated C files for the first mypyc run test failure in a
pytest session will be copied to the `.mypyc_test_output` directory, and
this will be indicated in the test output. This is a convenience feature
to help in the common scenario where all test failures have the same
root cause, so any single output is sufficient for debugging.

Document this and `--mypyc-showc`, which allows showing generated C for
every test failure. The latter is too verbose to be enabled by default.