add failing test, testReportIsADirectoryErrorCrashOnNamespacePackages

A regression test for the internal crash #19843

Author: wyattscarpenter

test-data/unit/reports.test

@@ -547,3 +547,11 @@ namespace_packages = True
 </table>
 </body>
 </html>
+
+[case testReportIsADirectoryErrorCrashOnNamespacePackages]
+# cmd: mypy --linecoverage-report report -p folder
+--Regression test for https://github.com/python/mypy/issues/18128
+-- "IsADirectoryError for namespace packages when using --linecoverage-report"
+[file folder/subfolder/something.py]
+class Something:
+    pass