cmd/go: add a few more calls to t.Parallel

Change-Id: If8fe5be9d2cd174862c09bb4ce079f524eb33ff3
Reviewed-on: https://go-review.googlesource.com/c/go/+/214580
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/go_test.go

@@ -2514,8 +2514,9 @@ func TestExecutableGOROOT(t *testing.T) {
 
 	// Note: Must not call tg methods inside subtests: tg is attached to outer t.
 	tg := testgo(t)
-	tg.unsetenv("GOROOT")
 	defer tg.cleanup()
+	tg.parallel()
+	tg.unsetenv("GOROOT")
 
 	check := func(t *testing.T, exe, want string) {
 		cmd := exec.Command(exe, "env", "GOROOT")

src/cmd/go/help_test.go

@@ -14,6 +14,8 @@ import (
 )
 
 func TestDocsUpToDate(t *testing.T) {
+	t.Parallel()
+
 	if !modload.Enabled() {
 		t.Skipf("help.Help in GOPATH mode is configured by main.main")
 	}

src/cmd/go/note_test.go

@@ -18,6 +18,8 @@ func TestNoteReading(t *testing.T) {
 	// both in internal and external linking mode.
 	tg := testgo(t)
 	defer tg.cleanup()
+	tg.parallel()
+
 	tg.tempFile("hello.go", `package main; func main() { print("hello, world\n") }`)
 	const buildID = "TestNoteReading-Build-ID"
 	tg.run("build", "-ldflags", "-buildid="+buildID, "-o", tg.path("hello.exe"), tg.path("hello.go"))

src/cmd/go/script_test.go

@@ -1246,6 +1246,8 @@ var diffTests = []struct {
 }
 
 func TestDiff(t *testing.T) {
+	t.Parallel()
+
 	for _, tt := range diffTests {
 		// Turn spaces into \n.
 		text1 := strings.ReplaceAll(tt.text1, " ", "\n")