runtime: if runtime is stale while testing, show cmd/go output

Update #19062.

Change-Id: If6a4c4f8d12e148b162256f13a8ee423f6e30637
Reviewed-on: https://go-review.googlesource.com/36918
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/crash_test.go

@@ -164,6 +164,7 @@ func checkStaleRuntime(t *testing.T) {
 			return
 		}
 		if string(out) != "false\n" {
+			t.Logf("go list -f {{.Stale}} runtime:\n%s", out)
 			staleRuntimeErr = fmt.Errorf("Stale runtime.a. Run 'go install runtime'.")
 		}
 	})