Fix tmux-multiline-prompt test with EDITOR=vim

This test does "isolated-tmux send-keys Escape" to exit copy mode. When
EDITOR contains "vi", tmux will use Vi keybindings where Escape does
something else ("q" would exit copy mode).

Tests want to have predictable behavior so let's declare the default
emacs key bindings unconditionally.

Fixes #10812

tests/test_functions/isolated-tmux-start.fish

@@ -2,9 +2,12 @@ function isolated-tmux-start
     set -l tmpdir (mktemp -d)
     cd $tmpdir
 
-    # macOS lacks the tmux-256color terminfo, use screen-256color instead.
-    if test (uname) = Darwin
-        echo 'set -g default-terminal "screen-256color"'
+    begin
+        echo 'set -g mode-keys emacs'
+        # macOS lacks the tmux-256color terminfo, use screen-256color instead.
+        if test (uname) = Darwin
+            echo 'set -g default-terminal "screen-256color"'
+        end
     end >./.tmux.conf
 
     function isolated-tmux --inherit-variable tmpdir