Add test_xmllint to rosidl_pycommon.

It is one of the tests that it is currently missing,
and will validate if the package.xml is bad for some reason.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

rosidl_pycommon/test/test_xmllint.py

@@ -0,0 +1,23 @@
+# Copyright 2019 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from ament_xmllint.main import main
+import pytest
+
+
+@pytest.mark.linter
+@pytest.mark.xmllint
+def test_xmllint() -> None:
+    rc = main(argv=[])
+    assert rc == 0, 'Found errors'