Check for <testsuites> vs <testsuite> root tag. Fixes #9

According to the JUnit XML specs:
https://www.ibm.com/support/knowledgecenter/en/SSUFAU_1.0.0/com.ibm.rsar.analysis.codereview.cobol.doc/topics/cac_useresults_junit.html

the root element in the XML is a <testsuites> tag. However some
of the test runners we've used skip that and have a single
<testsuite> tag as their root node. This causes failures for
people using some of the other test runners, e.g.

- Katalon Studio - exports with the <testsuites> tag
- Nose, py.test - export results with the <testsuite> tag

tcms_junit_plugin/__init__.py

@@ -14,7 +14,17 @@ def parse(self, junit_xml, progress_cb=None):
         self.backend.configure()
 
         xml = JUnitXml.fromfile(junit_xml)
-        for xml_case in xml:
+        # apparently junit.xml may contain either a <testsuites> tag - Katalon Studio
+        if xml._tag == "testsuites":
+            cases = []
+            for suite in xml:
+                for case in suite:
+                    cases.append(case)
+        # or directly <testsuite> (only 1) tag - nose & py.test
+        else:
+            cases = list(xml)
+
+        for xml_case in cases:
             summary = "%s.%s" % (xml_case.classname, xml_case.name)
 
             test_case = self.backend.test_case_get_or_create(summary)