fix: keep the host's XDG folders out of the tests

install.py reads RetroArch's base folder from XDG_CONFIG_HOME, and the
GitHub runner sets it: six tests that give the installer a home of
their own read the runner's ~/.config, failed in CI and passed here.

tests/__init__.py

@@ -17,6 +17,12 @@ import tempfile
 from pathlib import Path
 
 os.environ.setdefault("RETROBIOS_OFFLINE", "1")
+# The installer reads RetroArch's base folder from XDG_CONFIG_HOME as
+# RetroArch does. The GitHub runner sets it, and the tests that give the
+# installer a home of their own found the runner's ~/.config instead: six
+# failed in CI and passed here. A test that needs the variable sets it.
+for _xdg in ("XDG_CONFIG_HOME", "XDG_DATA_HOME"):
+    os.environ.pop(_xdg, None)
 # A test never waits on, nor blocks, the measurements running on the host.
 os.environ.setdefault(
     "RETROBIOS_PROBE_LOCK",