fix unit tests

tests/test_testing_tools.py

@@ -0,0 +1,30 @@
+import rio.testing
+
+
+async def test_crashed_build_functions_are_tracked():
+    def build() -> rio.Component:
+        return 3  # type: ignore
+
+    async with rio.testing.TestClient(build) as test_client:
+        assert len(test_client.crashed_build_functions) == 1
+
+
+async def test_rebuild_resets_crashed_build_functions():
+    class CrashingComponent(rio.Component):
+        fail: bool = True
+
+        def build(self) -> rio.Component:
+            if self.fail:
+                raise RuntimeError
+            else:
+                return rio.Text("hi")
+
+    async with rio.testing.TestClient(CrashingComponent) as test_client:
+        assert len(test_client.crashed_build_functions) == 1
+
+        crashing_component = test_client.get_component(CrashingComponent)
+        crashing_component.fail = False
+
+        await test_client.refresh()
+
+        assert not test_client.crashed_build_functions