tests: fix imports and test collection under some environments

[AluisioASG]

While packaging json-logging for NixOS, I've come across two issues with tests. Since neither are specific to NixOS (though they may be an artifact of how the build environment is set up), I felt it'd be better to try to stream the fixes up.

• While testing, the import test for Quart finds not the 'quart' package, but tests/smoketests/quart. It's been a while since I worked on this, but it seems to happen because pytest prepends the test module directory to sys.path by default. To make it work with a default pytest invocation, we can simply try to import something only the real quart package provides.
• If no smoketest backends are specified, it will try to find tests in a __pycache__ directory if it exists.

[bobbui]

@AluisioASG looks good. Thanks for your contribution