Don't mix every http request in with the tests output.
Don't print that the file servers are starting unless
-vv flag is passed.

Capture the output of run with run_output which returns
stdout, stderr and exit_code. Test against this rather
than relying on sys.exit.

Move every test to a method on DenoTestCase.
test.py is a single TestSuite of every TestCase.

Add a Spawn context manager for http_server,
this is explicitly used where it's needed.
Each python test file can now be run independently
without needing to manually run http_server.

Add --help and consistent flags using argparse for
each python test, including --failfast.

Use ColorTextTestRunner so that '... ok' is green.

Fixes #1742
Fixes #2021

Previously it would dump the traceback but not raise.
It's unclear if serve_forever could crash for some other reason,
but the main reason spawn throws is if the port is already in use.