Flaky visual test: Loading

[pathunstrom]

ppb commit: b2eee07a7f0ea OS Windows 10 Python 3.8.6

Output:

==========
loading.py
==========

Tests loading scenes.

Should take some time to progress. This may need to be run several times to suss
out threading bugs.

UserWarning: Using SDL2 binaries from pysdl2-dll 2.0.10
bullet.png 1.527485162932843
player.png 4.096429452946413
target.png 5.921888454316235
AssetLoaded(asset=<Image name='target.png' loaded at 0x149caf40220>, total_loaded=1, total_queued=0)
Traceback (most recent call last):
  File "C:\Users\pathu\PycharmProjects\pursuedpybear\viztests\loading.py", line 60, in <module>
    ppb.run(starting_scene=LoadingScene)
  File "c:\users\pathu\pycharmprojects\pursuedpybear\ppb\__init__.py", line 125, in run
    eng.run()
  File "c:\users\pathu\pycharmprojects\pursuedpybear\ppb\engine.py", line 311, in run
    self.main_loop()
  File "c:\users\pathu\pycharmprojects\pursuedpybear\ppb\engine.py", line 340, in main_loop
    self.loop_once()
  File "c:\users\pathu\pycharmprojects\pursuedpybear\ppb\engine.py", line 356, in loop_once
    self.publish()
  File "c:\users\pathu\pycharmprojects\pursuedpybear\ppb\engine.py", line 382, in publish
    method(event, self.signal)
  File "C:\Users\pathu\PycharmProjects\pursuedpybear\viztests\loading.py", line 37, in on_update
    assert self.bullet.is_loaded()
AssertionError
Traceback (most recent call last):
  File "C:\Users\pathu\AppData\Local\Programs\Python\Python38\lib\runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Users\pathu\AppData\Local\Programs\Python\Python38\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "C:\Users\pathu\PycharmProjects\pursuedpybear\viztests\__main__.py", line 24, in <module>
    subprocess.run([sys.executable, str(script)], check=True)
  File "C:\Users\pathu\AppData\Local\Programs\Python\Python38\lib\subprocess.py", line 512, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['C:\\Users\\pathu\\PycharmProjects\\pursuedpybear\\venv\\Scripts\\python.exe', 'C:\\Users\\pathu\\PycharmProjects\\pursuedpybe
ar\\viztests\\loading.py']' returned non-zero exit status 1.

Some kind of race condition I think? It's the assert failing.