Pytests can load a main file with runpy (#5102)

### Motivation

With the introduction of the root page concept, we need to rethink our
pytest infrastructure (he `module_under_test` marker and the modular
setup with a startup function)

### Implementation

`main.py` is loaded via `runpy.run_path` in the fixture setup.

### Progress

- [x] I chose a meaningful title that completes the sentence: "If
applied, this PR will..."
- [x] The implementation is complete.
- [x] Pytests have been added.
- [x] Documentation has been updated.

---------

Co-authored-by: Falko Schindler <[email protected]>

Author: rodja

examples/authentication/__init__.py

@@ -0,0 +1,4 @@
+[pytest]
+asyncio_mode = auto
+main_file = main.py
+addopts = -p nicegui.testing.plugin

examples/authentication/conftest.py

@@ -1,13 +1,8 @@
-import pytest
-
 from nicegui.testing import User
 
-from . import main
-
 # pylint: disable=missing-function-docstring
 
 
-@pytest.mark.module_under_test(main)
 async def test_login_logoff(user: User) -> None:
     await user.open('/')
     user.find('Username').type('user1')
@@ -18,15 +13,13 @@ async def test_login_logoff(user: User) -> None:
     await user.should_see('Log in')
 
 
-@pytest.mark.module_under_test(main)
 async def test_wrong_password(user: User) -> None:
     await user.open('/')
     user.find('Username').type('user1')
     user.find('Password').type('wrong').trigger('keydown.enter')
     await user.should_see('Wrong username or password')
 
 
-@pytest.mark.module_under_test(main)
 async def test_subpage_access(user: User) -> None:
     await user.open('/subpage')
     await user.should_see('Log in')

examples/authentication/pytest.ini

@@ -0,0 +1,4 @@
+[pytest]
+asyncio_mode = auto
+main_file = main.py
+addopts = -p nicegui.testing.plugin

examples/authentication/test_authentication.py

@@ -1,14 +1,9 @@
 from typing import Callable
 
-import pytest
-
 from nicegui import ui
 from nicegui.testing import User
 
-from . import main
-
 
-@pytest.mark.module_under_test(main)
 async def test_basic_startup_appearance(user: User) -> None:
     """Test basic appearance of the chat app."""
     await user.open('/')
@@ -18,7 +13,6 @@ async def test_basic_startup_appearance(user: User) -> None:
     await user.should_see('No messages yet')
 
 
-@pytest.mark.module_under_test(main)
 async def test_sending_messages(create_user: Callable[[], User]) -> None:
     """Test sending messages from two different screens."""
     userA = create_user()

examples/chat_app/__init__.py

@@ -1,2 +1,4 @@
 [pytest]
 asyncio_mode = auto
+main_file = main.py
+addopts = -p nicegui.testing.plugin