runtime: if runtime is stale while testing, show StaleReason

Update #19062.

Change-Id: I7397b573389145b56e73d2150ce0fc9aa75b3caa
Reviewed-on: https://go-review.googlesource.com/36934
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/crash_test.go

@@ -165,6 +165,11 @@ func checkStaleRuntime(t *testing.T) {
 		}
 		if string(out) != "false\n" {
 			t.Logf("go list -f {{.Stale}} runtime:\n%s", out)
+			out, err := testEnv(exec.Command(testenv.GoToolPath(t), "list", "-f", "{{.StaleReason}}", "runtime")).CombinedOutput()
+			if err != nil {
+				t.Logf("go list -f {{.StaleReason}} failed: %v", err)
+			}
+			t.Logf("go list -f {{.StaleReason}} runtime:\n%s", out)
 			staleRuntimeErr = fmt.Errorf("Stale runtime.a. Run 'go install runtime'.")
 		}
 	})