fuse integration: skip the permission-denied check as root

Root bypasses mode-bit checks, so writing the read-only file succeeded and
the assertion failed in root-run environments like containers.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

test/fuse_integration/posix_compliance_test.go

@@ -696,6 +696,9 @@ func (s *POSIXComplianceTestSuite) TestErrorHandling(t *testing.T) {
 	})
 
 	t.Run("PermissionDenied", func(t *testing.T) {
+		if os.Geteuid() == 0 {
+			t.Skip("running as root: mode bits are not enforced")
+		}
 		testFile := filepath.Join(mountPoint, "readonly.txt")
 
 		// Create read-only file