Tracking issue for `tests/` cleanup

[mmastrac]

• Split the test_util into the actual test utils and test_server, trim the deps on both of them to the minimum necessary
• Write docs in the tests/ directory describing each of the types of integration tests and how to run them
• Move tools/node_compat -> tests/node_compat/runner
• Move tools/wpt -> tests/wpt/runner
• Spawn the HTTP server from test_server at startup for the integration tests rather than using the httpguard

@iuioiua