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
Prior to this change, tmux based tests would call 'isolated-tmux' which would
initialize tmux on first call, an admitted "evil hack." Switch to requiring
an explicit call to 'isolated-tmux-start' which then defines 'isolated-tmux'
and other functions. Add some loop-until-prompt logic into
'isolated-tmux-start'. This improves reliability of the tmux tests on systems
under load; at least it makes the tests pass in the background on my Mac.
Remove the '$sleep' variable, to be replaced with 'tmux-sleep'.