regression test for #15979

#16019, which fixed this issue (for linux) was not accompanied by a regression test

Author: wyattscarpenter

test-data/unit/reports.test

@@ -547,3 +547,10 @@ namespace_packages = True
 </table>
 </body>
 </html>
+
+[case testLinecountReportCrashOnNamespacePackages]
+# cmd: mypy --linecount-report report -p folder
+--Regression test for https://github.com/python/mypy/issues/15979
+[file folder/subfolder/something.py]
+class Something:
+    pass