cmd/dist: don't run moved_goroot test on android, iOS, Plan 9

Fails on iOS because CC_FOR_TARGET points to clangwrap.sh in the
original GOROOT. We could fix that but it doesn't seem worth it.

Fails on Android with "exec format error". I'm not sure why but I
doubt it is interesting.

Fails on Plan 9 because the original GOROOT is being preserved in some
unknown way. This is issue #21016.

Updates #21016

Change-Id: I4e7115d734fc7bf21e5a2ba18fb6ad0bfa31c735
Reviewed-on: https://go-review.googlesource.com/48650
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/test.go

@@ -435,7 +435,11 @@ func (t *tester) registerTests() {
 	}
 
 	// On the builders only, test that a moved GOROOT still works.
-	if os.Getenv("GO_BUILDER_NAME") != "" {
+	// Fails on iOS because CC_FOR_TARGET refers to clangwrap.sh
+	// in the unmoved GOROOT.
+	// Fails on Android with an exec format error.
+	// Fails on plan9 with "cannot find GOROOT" (issue #21016).
+	if os.Getenv("GO_BUILDER_NAME") != "" && t.goos != "android" && !t.iOS() && t.goos != "plan9" {
 		t.tests = append(t.tests, distTest{
 			name:    "moved_goroot",
 			heading: "moved GOROOT",