Make the Dummy isolation provider follow the rest of the isolation
providers and perform the second part of the conversion on the host. The
first part of the conversion is just a dummy script that reads a file
from stdin and prints pixels to stdout.
Add a fixture that returns our stock Dummy provider. Also, explicitly
use a blocking Dummy provider (`DummyWait`) for a specific test case.
This will prove useful when we stop using the `provider_wait` variant of
our isolation providers in the next commits.
Start the conversion process in a new session, so that we can later on
kill the process group, without killing the controlling script (i.e.,
the Dangezone UI). This should not affect the conversion process in any
other way.
The platform where we run our tests directly affects the isolation
providers we can choose. For instance, we cannot run Qubes tests on a
Windows/macOS platform, nor can we spawn containers in a Qubes platform,
if the `QUBES_CONVERSION` envvar has been specified.
This platform incompatibility was never an issue before, because
Dangerzone is capable of selecting the proper isolation provider under
the hood. However, with the addition of tests that target specific
isolation providers, it's possible that we may run by mistake a test
that does not apply to our platform.
To counter this, we employed `pytest.skipif()` guards around classes,
but we may omit those by mistake. Case in point, the `TestContainer`
class does not have such a guard, which means that we attempt to run
this test case on Qubes and it fails.
Add module-level guards in our isolation provider tests using pytest's
`pytest.skip("...", allow_module_level=True)` function, so that we make
such restrictions more explicit, and less easy to forget when we add a
new class.
A few minor changes about when to use `==` and when to use `is`.
Basically, this uses `is` for booleans, and `==` for other values.
With a few other changes about coding style which was enforced by
`ruff`.
Add termination tests for the Dummy provider, so that we can have
cross-platform coverage in our Windows/macOS CI runners, which can't use
the Container isolation providers.
