run-test: act that we requires MSYS to run the test on Windows

If this ever change we can drop that constrains, however for now it seems clear
to be clear about this limitation.

Differential Revision: https://phab.mercurial-scm.org/D11080

tests/run-tests.py

@@ -3990,6 +3990,15 b' def aggregateexceptions(path):'
 
 
 if __name__ == '__main__':
+    if WINDOWS and not os.getenv('MSYSTEM'):
+        print('cannot run test on Windows without MSYSTEM', file=sys.stderr)
+        print(
+            '(if you need to do so contact the mercurial devs: '
+            '[e-mail])',
+            file=sys.stderr,
+        )
+        sys.exit(255)
+
     runner = TestRunner()
 
     try: