Add reload(module) test that checks the current module singleton property.

tests/run/reload_ext_module.pyx

+# mode: run
+# tag: pep489
+
+
+import reload_ext_module
+
+
+def test_reload(module):
+    """
+    >>> module = test_reload(reload_ext_module)
+    >>> module is reload_ext_module  # Py_mod_create enforces a singleton.
+    True
+    """
+    return reload(module)