Fix the expect tests under tmux by inspecting TERM

We were checking for the $TMUX variable to determine if we were
running under tmux. However when running the tests, the terminal becomes
expect, even though the TMUX variable is still set, so we spew tmux-isms
at expect. Check the value of $TERM for 'screen'.

share/functions/__fish_config_interactive.fish

@@ -239,6 +239,7 @@ function __fish_config_interactive -d "Initializations that should be performed
     # - The keybindings (reading the sequence and triggering an event)
     # - Any listeners (like the vi-cursor)
     if set -q TMUX
+       and [ "$TERM" = screen ]
         function __fish_enable_focus --on-event fish_postexec
             echo -n \e\[\?1004h
         end