cmd/guru: make tests granular and parallel
cmd/guru: make tests granular and parallel

This make it possible to easily run individual guru tests.

It also slightly speeds up the overall test run,
from 23s to 18s on my machine;
the TestGuru/calls is now the limiting factor,
by a significant margin.

This work supported by Sourcegraph.

Change-Id: If61ebf1cc60441a65274f3fddd31f69c7ca23b48
Reviewed-on: https://go-review.googlesource.com/108876
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

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