fix: clear the XDG folders for the test suite

The CI runner sets XDG_CONFIG_HOME to its own home, and the installer
now reads it as RetroArch does. Tests that fake HOME then found the
runner's RetroArch folders. The suite clears the three XDG variables
it reads, and a test that exercises one sets it itself.

tests/__init__.py

@@ -17,6 +17,12 @@ import tempfile
 from pathlib import Path
 
 os.environ.setdefault("RETROBIOS_OFFLINE", "1")
+# The installer reads the XDG base directories as RetroArch and the
+# emulators do. The CI runner sets XDG_CONFIG_HOME to its own home, so tests
+# that fake HOME found the runner's folders instead of theirs. A test that
+# exercises one of them sets it for itself.
+for _name in ("XDG_CONFIG_HOME", "XDG_DATA_HOME", "XDG_CACHE_HOME"):
+    os.environ.pop(_name, None)
 # A test never waits on, nor blocks, the measurements running on the host.
 os.environ.setdefault(
     "RETROBIOS_PROBE_LOCK",