Fix flaky auth tests

[nathanjcochran]

This fixes some flaky auth tests that were sometimes in failing in CI (for example, see here).

The issue manifested as an unexpected EOF from the local OAuth callback server, which was causing the tests to fail here when simulating a request from the user's browser:

--- FAIL: TestAuthLogin_OAuth_SingleProject (0.16s)
    auth_test.go:366: Mock browser opening URL: http://127.0.0.1:42739/oauth/authorize?client_id=45e1b16d-e435-4049-97b2-8daad150818c&code_challenge=QtzkvFiFpRa2QZlWnzHuvtn_Ad3qP3hCDl8wuKoXMkY&code_challenge_method=S256&redirect_uri=http%3A%2F%2Flocalhost%3A35441%2Fcallback&response_type=code&state=UsgJbg3qKUKKMhKgRKM1zY5wHZuv5Onp
    auth_test.go:419: Mock browser making callback request to: http://localhost:35441/callback?code=test-auth-code&state=UsgJbg3qKUKKMhKgRKM1zY5wHZuv5Onp
    auth_test.go:261: Mock server received token exchange request
    auth_test.go:423: Mock callback request failed: Get "http://localhost:35441/callback?code=test-auth-code&state=UsgJbg3qKUKKMhKgRKM1zY5wHZuv5Onp": EOF

I believe it was happening because we were previously doing a non-graceful shutdown of the local HTTP server that handles the callback request. The shutdown was happening after we had already returned a redirect and returned the result on the resultChan, so the code looked valid. However, because the http.ResponseWriter buffers data before flushing it to the underlying TCP connection, and we were doing a hard/non-graceful shutdown, the TCP connection was sometimes being closed before the redirect response had actually been flushed, resulting in an EOF for the caller. I heard no reports of this happening outside of CI, but it's possible it could have happened in the wild too.

This fixes it by doing a graceful shutdown (which waits for any in-progress requests to complete) instead of a hard shutdown.

[nathanjcochran]

This was just a confused attempt to fix the issue previously, but isn't actually needed. Because we open the net.Listener synchronously here, before we open the browser here, it should never be the case that the server isn't ready to handle incoming connections by the time this function executes.

[nathanjcochran]

I decided to get rid of this function entirely and just call http.Server.Shutdown() directly above. The shutdown method already closes the listener internally, so we shouldn't need to do it ourselves, like it was previously being done here.