The new directory structure will make extension easier.

Besides being the dedicated tool to assert on Qt signal emissions,
QSignalSpy can track when signals are emitted from other signals,
which the mocking strategy misses.

Example:

    some_signal.connect(some_other_signal)
    # verify that some_other_signal is emitted

See https://doc.qt.io/qt-5.15/qsignalspy.html

I'll use an expand and contract pattern to refactor
the export.Service tests, and keeping this file distinct
while it is used as a reference will be more convenient
and minimize the change sets.

Important: this file is still fully operational and will be
kept as such until all deprecated methods have been removed
from export.Export (soon to be renamed export.Service).

Refactor tests to:

- distinguish testing of GUI API, and CLI API internals
- improve readability

Decorate static methods (from what didn't need to be class methods)

Tests were minially modified: patching static methods must be done
unsing unittest.mock.patch.object to preserve the independence of
the unit tests. Assigning to the class attributes directly does
modify the class for the entire test session. (Not desirable.)

I am leaving the deprecated tests as untouched as possible to make
easier to review the changes.

This will ensure that one one instance of the export service
queries the sd-devices CLI.

That was already the case, but nothing was done to suggest it.

Additionally, this will allow to simplify the export service
injection by making possible to retrieve it like a logger.

Note: I haven't stopped injecting the export_service into
the main.Window because I couldn't figure out how to prevent
what seems to be a fixture scope error that affects only
half of the integration tests. That seems like something that
can be resolved, so I don't consider it to be a sign of
export.Service being a bad idea.

…ce (1)

These are now replaced by "printer check" and "disk check" which
have the advantage of referring to concrete physical devices.

…ce (2)

…ce (3)

Rename disk functions in export.CLI and export.Service
Rename printer functions in export.CLI and export.Service

The CLI now encapsulates all "preflight" references, so that
neither the CLI or the Service expose them publicly.
(References remain on the export.Device and dialogs, but those
can now be removed for consitency with the export.Service.)

Refactor remove unnecessary methods that were already deprecated

Note: some of the deprecated tests were testing private methods
that are currectly CLI behavior. This commit moves that testing to
the appropriate file.

With the new separation of concerns, the behavior is tested in three
differemt component:

- the service, that exposes the public API for the export module
- the CLI that interfaces the service and Qubes OS tooling
- the archive that is the CLI relies on but has no relation to Qubes OS

The export.getService() function allows to obtain the export service
instance wherever necessary without needing to inject it through all
the parent widgets.

This is a double-edged sword, as it makes the injection more implicit,
but it is a pattern that is commonly used for singletons, e.g. loggers.
Use thoughtfully, and remember that injecting the export service
explicitly in some widgets is always possible!

Next steps: as soon as the export.Device is not longer necessary, then
the export.Service type doesn't need to be exported anymore.

This interface allows to replace the export.Device (deprecated)
by the export.Disk in the export dialog with minimal changes.

It shouldn't be used for any new developments.

The native errors are already available as disk.last_error.

Resetting the export.Service as part of the setup of the tests is
a compromise. It is a smell, that accurately points at other tests
not performing an adequate teardown.

I haven't fixed those this time around because the way they are written
doesn't provide an obvious place to teardown related tests, so adding
teardown operations introduces a significant amount of duplication.

It seems to me that athough less than ideal, this will prove more
reliable and more less error-prone for the time being.