Back out "Remove test for broken tmux output"

Add back the test from 0e512f8033 (Fix spurious blank lines when
executing scrolled commandline, 2025-01-08).

It's flaky in CI (see b8208d72f7 (Remove test for broken tmux output,
2025-01-09)) but that's not a problem for now because we disabled
the entire test CI in 2508cc9de6 (Disable some more tests under CI,
2025-01-14).

tests/checks/tmux-commandline.fish

@@ -40,6 +40,23 @@ isolated-tmux capture-pane -p
 # CHECK: 10
 # CHECK: scroll_here
 
+# The output is broken here (seems tmux specific).
+isolated-tmux send-keys C-c
+tmux-sleep
+isolated-tmux send-keys C-l 'commandline -i ": \'$(seq $LINES)" A B "C\'"' Enter Enter
+tmux-sleep
+isolated-tmux capture-pane -p
+# CHECK: 4
+# CHECK: 5
+# CHECK: 6
+# CHECK: 7
+# CHECK: 8
+# CHECK: 9
+# CHECK: 10
+# CHECK: prompt 5>
+# CHECK: B
+# CHECK: C'
+
 # Soft-wrapped commandline with omitted right prompt.
 isolated-tmux send-keys C-q '
     function fish_right_prompt