cmd/go: add detail to test failures

For #50892

Change-Id: I14ff1c43b39687a0ba5e668ee962cecfb49e4beb
Reviewed-on: https://go-review.googlesource.com/c/go/+/381836
Trust: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: Bryan C. Mills

src/cmd/go/go_test.go

@@ -133,7 +133,7 @@ func TestMain(m *testing.M) {
 		}
 		gotool, err := testenv.GoTool()
 		if err != nil {
-			fmt.Fprintln(os.Stderr, err)
+			fmt.Fprintln(os.Stderr, "locating go tool: ", err)
 			os.Exit(2)
 		}
 

src/cmd/go/script_test.go

@@ -902,7 +902,7 @@ func (ts *testScript) cmdStale(want simpleStatus, args []string) {
 	tmpl := "{{if .Error}}{{.ImportPath}}: {{.Error.Err}}{{else}}"
 	switch want {
 	case failure:
-		tmpl += "{{if .Stale}}{{.ImportPath}} is unexpectedly stale{{end}}"
+		tmpl += "{{if .Stale}}{{.ImportPath}} is unexpectedly stale: {{.StaleReason}}{{end}}"
 	case success:
 		tmpl += "{{if not .Stale}}{{.ImportPath}} is unexpectedly NOT stale{{end}}"
 	default: