Improve testing of COW/Fetch

[james-pre]

Right now, tests/setup/cow+fetch.ts handles creating a HTTP server. Since it is run for every test suite/file, it gracefully continues if the server is already running. Sometimes, there is a small amount of time when a server is not running but a test is, which causes it to fail.

This can be fixed by having the server run externally from the test suites, meaning only one server would run for all of them. The change will also have the tests running a lot faster.