runtime: TestSetGCPercent flaky

[josharian]

Despite CL 41374, I'm seeing a bunch of trybot failures like https://storage.googleapis.com/go-build-log/6c83b9bd/linux-amd64_cce08b9e.log:

--- FAIL: TestSetGCPercent (0.17s)
	garbage_test.go:151: NextGC = 172 MB, want 150±20 MB

cc @aclements

[ALTree]

This is failing a lot. Just for reference: I see CL 41374 bumped from ±10 to ±20. Bumping again to ±30 won't help because I've seen failures with 181 (+31).

[gopherbot]

CL https://golang.org/cl/41615 mentions this issue.