proposal: rework and fix std/http/file_server and tests

[iuioiua]

std/http/file_server tests are currently flaky. One reason this may be happening is that each of these tests opens a new process and makes fetch() calls. However, such network calls and new processes are not needed. Instead, we can use something like const res = await serveDir(req). This would be a far more straightforward method of testing.