Strengthen wait test to verify fzf stays alive after cancel

Assert that a query still filters after Ctrl-C, proving fzf keeps
running and accepts input. The disappearing indicator alone would also
pass if fzf exited.

test/test_core.rb

@@ -1168,6 +1168,9 @@ class TestCore < TestInteractive
     tmux.until { |lines| assert lines.any_include?('20/100 (..)') }
     tmux.send_keys 'C-c'
     tmux.until { |lines| refute lines.any_include?('20/100 (..)') }
+    # Ctrl-C cancels the wait; fzf keeps running and accepts input again
+    tmux.send_keys '99'
+    tmux.until { |lines| assert_equal 1, lines.match_count }
   end
 
   def test_clear_selection