Move mock_serial patch to dedicated file

tests/mocks/mock_serial_patch.py

@@ -0,0 +1,35 @@
+import threading
+import time
+
+from mock_serial.mock_serial import Stub
+
+
+class WaitableStub(Stub):
+    """
+    In some situations, a test might be checking if a stub has been called before `MockSerial` thread had time
+    to read, match, and call the stub. In these situations, the test can fail randomly.
+
+    Use `wait_called()` or `wait_calls()` to block until the stub is called, avoiding race conditions.
+
+    Proposed fix:
+    https://github.com/benthorner/mock_serial/pull/3
+    """
+
+    def __init__(self, **kwargs):
+        super().__init__(**kwargs)
+        self._event = threading.Event()
+
+    def call(self):
+        self._event.set()
+        return super().call()
+
+    def wait_called(self, timeout: float = 1.0):
+        return self._event.wait(timeout)
+
+    def wait_calls(self, min_calls: int = 1, timeout: float = 1.0):
+        start = time.perf_counter()
+        while time.perf_counter() - start < timeout:
+            if self.calls >= min_calls:
+                return self.calls
+            time.sleep(0.005)
+        raise TimeoutError(f"Stub not called {min_calls} times within {timeout} seconds.")