xdist fixes (microsoft#23791)

- Don't use xdist when only running one test. this makes it slightly
faster when running one test, because instead of creating a single
worker it just runs the test in the same process
- fix microsoft#23816
- fix issue where the plugin was being registered multiple times

---------

Co-authored-by: detachhead <detachhead@users.noreply.github.com>

Author: DetachHead

python_files/vscode_pytest/__init__.py

@@ -19,6 +19,7 @@
 )
 
 import pytest
+from pluggy import Result
 
 script_dir = pathlib.Path(__file__).parent.parent
 sys.path.append(os.fspath(script_dir))
@@ -889,11 +890,25 @@ def send_post_request(
 
 class DeferPlugin:
     @pytest.hookimpl(hookwrapper=True)
-    def pytest_xdist_auto_num_workers(self, config: pytest.Config) -> Generator[None, int, int]:
+    def pytest_xdist_auto_num_workers(
+        self, config: pytest.Config
+    ) -> Generator[None, Result[int], None]:
         """Determine how many workers to use based on how many tests were selected in the test explorer."""
-        return min((yield), len(config.option.file_or_dir))
+        outcome = yield
+        result = min(outcome.get_result(), len(config.option.file_or_dir))
+        if result == 1:
+            result = 0
+        outcome.force_result(result)
 
 
 def pytest_plugin_registered(plugin: object, manager: pytest.PytestPluginManager):
-    if manager.hasplugin("xdist") and not isinstance(plugin, DeferPlugin):
-        manager.register(DeferPlugin())
+    plugin_name = "vscode_xdist"
+    if (
+        # only register the plugin if xdist is enabled:
+        manager.hasplugin("xdist")
+        # prevent infinite recursion:
+        and not isinstance(plugin, DeferPlugin)
+        # prevent this plugin from being registered multiple times:
+        and not manager.hasplugin(plugin_name)
+    ):
+        manager.register(DeferPlugin(), name=plugin_name)