tstest: don't log on success

This is particularly important for benchmarks,
where stray output disrupts tooling.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tstest/resource.go

@@ -68,5 +68,4 @@ func (r *ResourceCheck) Assert(t testing.TB) {
 			}
 		}
 	}
-	t.Logf("ResourceCheck ok: goroutines before=%d after=%d\n", got, want)
 }