ipn/ipnlocal: reenable profile tests on Windows

This fix does not seem ideal, but the test infrastructure using a local
goos doesn't seem to avoid all of the associated challenges, but is
somewhat deeply tied to the setup.

The core issue this addresses for now is that when run on Windows there
can be no code paths that attempt to use an invalid UID string, which on
Windows is described in [1].

For the goos="linux" tests, we now explicitly skip the affected
migration code if runtime.GOOS=="windows", and for the Windows test we
explicitly use the running users uid, rather than just the string
"user1". We also now make the case where a profile exists and has
already been migrated a non-error condition toward the outer API.

Updates #7876

[1] https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/manage/understand-security-identifiers

Signed-off-by: James Tucker <jftucker@gmail.com>

ipn/ipnlocal/profiles_windows.go

@@ -6,6 +6,7 @@ package ipnlocal
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"os/user"
 	"path/filepath"
@@ -21,8 +22,6 @@ const (
 	legacyPrefsExt                   = ".conf"
 )
 
-var errAlreadyMigrated = errors.New("profile migration already completed")
-
 func legacyPrefsDir(uid ipn.WindowsUserID) (string, error) {
 	// TODO(aaron): Ideally we'd have the impersonation token for the pipe's
 	// client and use it to call SHGetKnownFolderPath, thus yielding the correct
@@ -56,6 +55,9 @@ func (pm *profileManager) loadLegacyPrefs() (string, ipn.PrefsView, error) {
 
 	prefsPath := filepath.Join(userLegacyPrefsDir, legacyPrefsFile+legacyPrefsExt)
 	prefs, err := ipn.LoadPrefs(prefsPath)
+	if errors.Is(err, fs.ErrNotExist) {
+		return "", ipn.PrefsView{}, errAlreadyMigrated
+	}
 	if err != nil {
 		return "", ipn.PrefsView{}, err
 	}