fix(trash): keep tests off the machine's real per-mount trash dirs (#3100)

setupHomeTrash redirects HOME and XDG_DATA_HOME, but allTrashDirs also
walks readMountPoints() looking for <mount>/.Trash/<uid> and
<mount>/.Trash-<uid>, and no environment variable affects that walk. So
Count() and List() included whatever the developer had trashed on a
second filesystem, and Empty() deleted it for real: with an orphaned
file under a mount trash (no matching .trashinfo, so the count assert
still saw 3), TestEmptyClearsAll passed and removed the file.

Route the walk through a package-level mountPoints variable and have
setupHomeTrash swap in an empty list, restoring it via t.Cleanup.
Production behavior is unchanged.

core/internal/trash/trash.go

@@ -250,6 +250,10 @@ func Put(path string) (Entry, error) {
 	}, nil
 }
 
+// mountPoints is a variable so tests can keep the mount walk off the real
+// filesystem; HOME and XDG_DATA_HOME alone do not redirect it.
+var mountPoints = readMountPoints
+
 // allTrashDirs returns the home trash plus every per-mountpoint trash dir
 // that exists (and passes the spec's safety checks for $topdir/.Trash).
 func allTrashDirs() []string {
@@ -259,7 +263,7 @@ func allTrashDirs() []string {
 	}
 
 	uid := strconv.Itoa(os.Getuid())
-	for _, mount := range readMountPoints() {
+	for _, mount := range mountPoints() {
 		shared := filepath.Join(mount, ".Trash")
 		if isValidSharedTrash(shared) {
 			candidate := filepath.Join(shared, uid)

core/internal/trash/trash_test.go

@@ -17,6 +17,13 @@ func setupHomeTrash(t *testing.T) (homeRoot string, trashDir string) {
 	}
 	t.Setenv("XDG_DATA_HOME", xdg)
 	t.Setenv("HOME", homeRoot)
+
+	// Without this, Count() and Empty() reach the machine's real per-mount
+	// trash dirs, and Empty() deletes what it finds there.
+	prev := mountPoints
+	mountPoints = func() []string { return nil }
+	t.Cleanup(func() { mountPoints = prev })
+
 	trashDir = filepath.Join(xdg, "Trash")
 	return homeRoot, trashDir
 }