Add more detailed test for fetching sourcemap from multiple fleet servers

[estolfo]

As per this comment, add a more detailed test for fetching sourcemap from multiple fleet servers when some fail and some succeed.

• call fetch in a goroutine
• block until all servers have received their request
• have ts0 respond with failure
• have ts1 respond with success
• block until ts2's request is cancelled

[stuartnelson3]

Was taking a look at this, and I don't think there's a way to block within the test code until ts2's request has been canceled. As the fetching code is written, as soon as we get a successful response, we return to the caller. We have no way to know, from the test server's perspective, if the client we're writing to is still reading on the other side. Additionally, even though the fetching code has returned, the test server is still able to successfully write a response.

I added a test which blocks until all test servers have received their requests, and then verifies that the fetching code returns after a successful request, but before the 3rd (and final) test server handler has finished executing. I was able to verify via print statements that the 3rd request is being canceled and the results channel being ranged over is successfully closed.