Include <testsuites> root tag in generated XML

Fix #5477

src/_pytest/junitxml.py

@@ -657,18 +657,17 @@ class LogXML:
         )
         logfile.write('<?xml version="1.0" encoding="utf-8"?>')
 
-        logfile.write(
-            Junit.testsuite(
-                self._get_global_properties_node(),
-                [x.to_xml() for x in self.node_reporters_ordered],
-                name=self.suite_name,
-                errors=self.stats["error"],
-                failures=self.stats["failure"],
-                skipped=self.stats["skipped"],
-                tests=numtests,
-                time="%.3f" % suite_time_delta,
-            ).unicode(indent=0)
+        suite_node = Junit.testsuite(
+            self._get_global_properties_node(),
+            [x.to_xml() for x in self.node_reporters_ordered],
+            name=self.suite_name,
+            errors=self.stats["error"],
+            failures=self.stats["failure"],
+            skipped=self.stats["skipped"],
+            tests=numtests,
+            time="%.3f" % suite_time_delta,
         )
+        logfile.write(Junit.testsuites([suite_node]).unicode(indent=0))
         logfile.close()
 
     def pytest_terminal_summary(self, terminalreporter):