x/tools: disable tests not supported on Android

For golang/go#11811.

Change-Id: Icf16a2d47fcf2fe0d79dd825ccb851a3d63a660f
Reviewed-on: https://go-review.googlesource.com/13268
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

vcs_test.go

@@ -9,13 +9,18 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"runtime"
 	"strings"
 	"testing"
 )
 
 // Test that RepoRootForImportPath creates the correct RepoRoot for a given importPath.
 // TODO(cmang): Add tests for SVN and BZR.
 func TestRepoRootForImportPath(t *testing.T) {
+	if runtime.GOOS == "android" {
+		t.Skipf("incomplete source tree on %s", runtime.GOOS)
+	}
+
 	tests := []struct {
 		path string
 		want *RepoRoot