cmd/godoc: check if server exited when waiting
cmd/godoc: check if server exited when waiting

Previously, the waitForServer family of helpers would wait anywhere
between 15 seconds to 2 minutes for the server to become ready.
But if there's a problem that results in the server exiting early,
that wasn't being detected quickly.

This change modifies tests to also wait for command to exit,
and fail the test quickly if so. This helps during development.

Updates golang/go#33655

Change-Id: I16195715449015d7250a2d0de5e55ab9a1ef078d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/196979
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools