test: fix cli test which fails only under lxml

under `lxml` the `encoding` property "UTF-8" is written in capslock, without `lxml` its lowercase.
So we ignore that property and only test for the "<?xml version='1.0'" part

tests/test_cli.py

@@ -5,7 +5,6 @@
 
 DATA_DIR = Path(__file__).parent / "data"
 
-
 @pytest.mark.parametrize(
     "file, expected_exitcode",
     [("jenkins.xml", 1), ("no_fails.xml", 0), ("normal.xml", 1)],
@@ -29,7 +28,7 @@ def test_merge_output_to_terminal(capsys: pytest.CaptureFixture):
     ret = cli.main(["merge", str(DATA_DIR / "normal.xml"), "-"])
     assert ret == 0
     captured = capsys.readouterr()
-    assert captured.out.startswith("<?xml version='1.0' encoding='utf-8'?>")
+    assert captured.out.startswith("<?xml version='1.0'")
 
 
 def test_verify_with_glob():