fuse integration: clean TEST_MOUNT_POINT before the parent-device check

filepath.Dir of a trailing-slash path returns the mount itself, so a mounted TEST_MOUNT_POINT ending in / was refused.

test/fuse_integration/mounted_unix.go

@@ -17,8 +17,10 @@ func deviceID(path string) (uint64, error) {
 }
 
 // mountedAt reports whether something is actually mounted at path: a plain
-// directory shares its parent's device, a mount point does not.
+// directory shares its parent's device, a mount point does not. The path is
+// cleaned first so a trailing slash does not make Dir return the mount itself.
 func mountedAt(path string) bool {
+	path = filepath.Clean(path)
 	dev, err := deviceID(path)
 	if err != nil {
 		return false

test/fuse_integration/mounted_unix_test.go

@@ -0,0 +1,19 @@
+//go:build unix
+
+package fuse
+
+import "testing"
+
+func TestMountedAt(t *testing.T) {
+	for _, p := range []string{"/proc", "/proc/"} {
+		if !mountedAt(p) {
+			t.Errorf("mountedAt(%q) = false, want true", p)
+		}
+	}
+	plain := t.TempDir()
+	for _, p := range []string{plain, plain + "/"} {
+		if mountedAt(p) {
+			t.Errorf("mountedAt(%q) = true, want false", p)
+		}
+	}
+}