runtime/debug: SetGCPercent(-1) doesn't wait for concurrent GC

[aclements]

Several runtime tests use SetGCPercent(-1) to disable GC because they're about to do something that interferes with preemption (sigh, #10958). However, now that SetGCPercent no longer forces a full GC, it also doesn't force a currently running concurrent GC to terminate. This results in the following possible situation:

1. Concurrent GC starts. Sweep termination runs and we enter concurrent mark.
2. Test calls SetGCPercent(-1).
3. Test enters a non-preemptible loop.
4. Concurrent GC tries to STW to enter mark termination, but can't preempt the loop, so the test deadlocks.

This can be reproduced pretty quickly by running cd runtime; go test -c; GOGC=5 stress -p 1 ./runtime.test -test.short.

Since the point of SetGCPercent(-1) is to disabled GC, it should probably want for any concurrent GC cycle to finish.

/cc @RLH

[gopherbot]

Change https://golang.org/cl/79195 mentions this issue: runtime/debug: make SetGCPercent(-1) wait for concurrent GC