bpo-36871: Avoid duplicated 'Actual:' in assertion message (GH-16361)

Fixes an issue caught after merge of PR 16005.

Tightened test assertions to check the entire assertion message.

Lib/unittest/mock.py

@@ -939,8 +939,8 @@ class NonCallableMock(Base):
                                     for e in expected])
                 raise AssertionError(
                     f'{problem}\n'
-                    f'Expected: {_CallList(calls)}\n'
-                    f'Actual: {self._calls_repr(prefix="Actual")}'
+                    f'Expected: {_CallList(calls)}'
+                    f'{self._calls_repr(prefix="Actual").rstrip(".")}'
                 ) from cause
             return