ui, tests: isolate HOME and XDG dirs from the real installation

[aadrian]

Running the UI tests on a machine where OpenSnitch is installed changes the settings of that installation:

• test_preferences.py deletes ~/.config/opensnitch/settings.conf, and the dialogs then rewrite it through QSettings("opensnitch", "settings").
• Pressing Apply in the preferences tests calls Autostart.enable(). That can create or remove ~/.config/autostart/opensnitch_ui.desktop, or add Hidden=true to it, which turns off autostart of the installed UI.

I noticed this after a local test run reset the settings of the UI I was running.

This PR only changes ui/tests/conftest.py:

• HOME and XDG_* point to a temporary directory. It is set up before opensnitch is imported, because utils/xdg.py reads these variables at import time. DBUS_SESSION_BUS_ADDRESS is unset, so no notification reaches the desktop.
• A session fixture fails if the settings, autostart or runtime paths resolve outside that directory. It also fails if the real settings.conf or autostart file changed during the run.

Tested:

• ui/tests gives the same result as master: 94 passed, 4 failed. The 4 failures are in test_preferences.py, are unrelated, and already happen on master. The real files were byte-identical before and after the run.
• A test that writes to a guarded file makes the run error with test run modified the real installation.
• It merges cleanly with cli: add opensnitch-cli, a headless client for machines with no graphical environment #1650 and Use ClientConfig.node_id as canonical UI node identity #1578, which also touch conftest.py. PyQt6 is only imported inside the fixture, so the CLI tests of cli: add opensnitch-cli, a headless client for machines with no graphical environment #1650, which run without Qt, keep working.

Several open PRs add or change UI tests (#1650, #1614, #1578, ...). Merging this first would keep anyone who runs them locally from losing their settings.

[aadrian]

@gustavo-iniguez-goya in case it helps with 1.9.0: these four open PRs fix things that break the UI or the package builds on a clean install. Each is small and based on master.

• ui, plugins: fixed loading versionchecker and downloader #1680: the UI doesn't start with the versionchecker or downloader plugin enabled (they import the removed opensnitch.dialogs.stats).
• ui, plugins: fixed loading list_subscriptions with protobuf < 3.20 #1684: the UI doesn't start with the list_subscriptions plugin on protobuf < 3.20 (Fedora 44 has 3.19.6). Reported in [Bug Report] list_subscriptions plugin prevents the UI from starting with protobuf < 3.20 (Fedora 44) #1683.
• ui, packaging: fixed dependencies for 1.9.0 #1685: python3-requests is missing from the deb/rpm dependencies, and clean deb builds fail because of undeclared build dependencies and the pyrcc5 step.
• ui,i18n: added missing sources to the .pro file #1682: opensnitch_i18n.pro is missing sources, so the next make would drop the existing pop-up translations of 20 locales.

Thanks in advance.