refactor(testing): use directory target, keep generated-driver guard

The dir->file change in 17d368b was not what fixed Linux (the window-realize
/ ready-to-show fixes were; both the dir and file forms reported 'No tests were
found' until then). Revert the flutter test target to the directory form
('integration_test') and keep only the useful part: in device mode, validate
the generated integration_test/app_test.dart exists and is non-empty so a
missing/empty driver surfaces as a clear error instead of a confusing
'No tests were found'.

Author: FeodorFitsner

sdk/python/packages/flet/src/flet/testing/flet_test_app.py

@@ -337,22 +337,25 @@ def connected() -> bool:
                 )
 
     def __flutter_test_target(self) -> str:
-        if not self.__device_mode:
-            return "integration_test"
-
-        app_test_path = (
-            Path(self.__flutter_app_dir) / "integration_test" / "app_test.dart"
-        )
-        if app_test_path.is_file():
+        # In device mode the driver (`integration_test/app_test.dart`) is
+        # generated from the template; validate it exists and is non-empty so a
+        # missing/empty driver surfaces as a clear error instead of a confusing
+        # "No tests were found" from `flutter test`. The directory target is
+        # used either way.
+        if self.__device_mode:
+            app_test_path = (
+                Path(self.__flutter_app_dir) / "integration_test" / "app_test.dart"
+            )
+            if not app_test_path.is_file():
+                raise RuntimeError(
+                    f"Flutter integration test driver was not generated: "
+                    f"{app_test_path}"
+                )
             if not app_test_path.read_text(encoding="utf-8").strip():
                 raise RuntimeError(
                     f"Flutter integration test driver is empty: {app_test_path}"
                 )
-            return str(Path("integration_test") / "app_test.dart")
-
-        raise RuntimeError(
-            f"Flutter integration test driver was not generated: {app_test_path}"
-        )
+        return "integration_test"
 
     async def teardown(self):
         """

sdk/python/packages/flet/tests/test_flet_test_app.py

@@ -1,20 +1,17 @@
-from pathlib import Path
-
 import pytest
 
 from flet.testing.flet_test_app import FletTestApp
 
 
-def test_flutter_test_target_uses_generated_app_test(tmp_path):
+def test_flutter_test_target_validates_generated_app_test_in_device_mode(tmp_path):
     app_test = tmp_path / "integration_test" / "app_test.dart"
     app_test.parent.mkdir()
     app_test.write_text("void main() {}\n", encoding="utf-8")
 
     flet_app = FletTestApp(flutter_app_dir=tmp_path, device_mode=True)
 
-    assert flet_app._FletTestApp__flutter_test_target() == str(
-        Path("integration_test") / "app_test.dart"
-    )
+    # The directory target is used; the generated driver is only validated.
+    assert flet_app._FletTestApp__flutter_test_target() == "integration_test"
 
 
 def test_flutter_test_target_requires_generated_app_test_in_device_mode(tmp_path):