Fix pytest junitxml output processing for make check

Not every element tagged `skipped` in the JUnitXML tree has to contain
the `type` attribute. An example of that is a test that results in
xpass.

This has been verified with pytest version 7.4.2 and prior.

bin/tests/system/convert-junit-to-trs.py

@@ -34,7 +34,7 @@ def junit_to_trs(junit_xml):
                 res = "ERROR"
                 has_error = True
             elif node.tag == "skipped":
-                if node.attrib["type"] == "pytest.xfail":
+                if node.attrib.get("type") == "pytest.xfail":
                     res = "XFAIL"
                 else:
                     res = "SKIP"