tests/tmux: Unset CDPATH

This would fail in tmux_complete

tests/test_functions/isolated-tmux-start.fish

@@ -49,6 +49,7 @@ function isolated-tmux-start --wraps fish
         set fish_history ""
         # No transient prompt.
         set fish_transient_prompt 0
+        set -g CDPATH
     ' $argv
     # Set the correct permissions for the newly created socket to allow future connections.
     # This is required at least under WSL or else each invocation will return a permissions error.