go/packages/packagestest: make Export skip tests involving unsupporte…

…d links

Also make Export create all parent directories before all files.
If the files are symlinks to directories, the target directory must
exist before the symlink is created on Windows. Otherwise, the symlink
will be created with the wrong type (and thus broken).

Fixes golang/go#46503
Updates golang/go#38772
Updates golang/go#39183

Change-Id: I17864ed66e0464e0ed1f56c55751b384b8c59484
Reviewed-on: https://go-review.googlesource.com/c/tools/+/234112
Trust: Bryan C. Mills <bcmills@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/imports/fix_test.go

@@ -11,7 +11,6 @@ import (
 	"io/ioutil"
 	"log"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"sync"
 	"testing"
@@ -1291,11 +1290,6 @@ func bar() {
 // Test support for packages in GOPATH that are actually symlinks.
 // Also test that a symlink loop does not block the process.
 func TestImportSymlinks(t *testing.T) {
-	switch runtime.GOOS {
-	case "windows", "plan9":
-		t.Skipf("skipping test on %q as there are no symlinks", runtime.GOOS)
-	}
-
 	const input = `package p
 
 var (
@@ -1331,11 +1325,6 @@ var (
 }
 
 func TestImportSymlinksWithIgnore(t *testing.T) {
-	switch runtime.GOOS {
-	case "windows", "plan9":
-		t.Skipf("skipping test on %q as there are no symlinks", runtime.GOOS)
-	}
-
 	const input = `package p
 
 var (