Fix tests by setting XDG_CACHE_HOME

We use this now, so need to have it set to the test homedir

tests/libtest.sh

@@ -80,6 +80,7 @@ mkdir -p ${TEST_DATA_DIR}/system
 export FLATPAK_SYSTEM_DIR=${TEST_DATA_DIR}/system
 export FLATPAK_SYSTEM_HELPER_ON_SESSION=1
 
+export XDG_CACHE_HOME=${TEST_DATA_DIR}/home/cache
 export XDG_DATA_HOME=${TEST_DATA_DIR}/home/share
 export XDG_RUNTIME_DIR=${TEST_DATA_DIR}/runtime