Move generated completions to cache directory

tests/test_env.sh

@@ -50,6 +50,10 @@ export XDG_RUNTIME_DIR
 mkdir -p $XDG_RUNTIME_DIR/fish || die
 chmod 700 "$XDG_RUNTIME_DIR"
 
+XDG_CACHE_HOME="$homedir/xdg_cache_home"
+export XDG_CACHE_HOME
+mkdir -p $XDG_CACHE_HOME/fish || die
+
 # Create a temp/scratch directory for tests to use, if they want (tests shouldn't write to a
 # shared temp folder).
 TMPDIR="$homedir/temp"