isolated-tmux to more reliably initialize and other fixes

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'.

tests/checks/tmux-bind.fish

@@ -1,9 +1,11 @@
 #RUN: %fish %s
 #REQUIRES: command -v tmux
 
+isolated-tmux-start
+
 # Test moving around with up-or-search on a multi-line commandline.
 isolated-tmux send-keys 'echo 12' M-Enter 'echo ab' C-p 345 C-n cde
-$sleep
+tmux-sleep
 isolated-tmux capture-pane -p
 # CHECK: prompt 0> echo 12345
 # CHECK: echo abcde

tests/checks/tmux-complete.fish

@@ -1,13 +1,13 @@
 #RUN: %fish %s
 #REQUIRES: command -v tmux
 
-isolated-tmux
+isolated-tmux-start
 
 # Don't escape existing token (#7526).
 echo >file-1
 echo >file-2
 isolated-tmux send-keys 'HOME=$PWD ls ~/' Tab
-$sleep
+tmux-sleep
 isolated-tmux capture-pane -p
 # Note the contents may or may not have the autosuggestion appended - it is a race.
 # CHECK: prompt 0> HOME=$PWD ls ~/file-{{1?}}
@@ -16,14 +16,14 @@ isolated-tmux capture-pane -p
 # No pager on single smartcase completion (#7738).
 isolated-tmux send-keys C-u C-l 'mkdir cmake CMakeFiles' Enter C-l \
     'cat cmake' Tab
-$sleep
+tmux-sleep
 isolated-tmux capture-pane -p
 # CHECK: prompt 1> cat cmake/
 
 # Correct case in pager when prefixes differ in case (#7743).
 isolated-tmux send-keys C-u C-l 'complete -c foo2 -a "aabc aaBd" -f' Enter C-l \
     'foo2 A' Tab
-$sleep
+tmux-sleep
 isolated-tmux capture-pane -p
 # The "bc" part is the autosuggestion - we could use "capture-pane -e" to check colors.
 # CHECK: prompt 2> foo2 aabc

tests/checks/tmux-prompt.fish

@@ -8,20 +8,13 @@ set -g isolated_tmux_fish_extra_args -C '
     end
 '
 
-# Start it up and loop a bit, until we get an initial prompt.
-isolated-tmux
-for i in seq 10
-    if string match -q '*prompt*' (isolated-tmux capture-pane -p)
-        break
-    end
-    sleep .5
-end
+isolated-tmux-start
 
 isolated-tmux capture-pane -p
 # CHECK: prompt 0> <>
 
 set -q CI && set sleep sleep 10
 set -U prompt_var changed
-$sleep
+tmux-sleep
 isolated-tmux capture-pane -p
 # CHECK: prompt 0> <changed>

tests/test_functions/isolated-tmux-start.fish

@@ -1,14 +1,7 @@
-function isolated-tmux --inherit-variable tmpdir
+function isolated-tmux-start
     set -l tmpdir (mktemp -d)
     cd $tmpdir
 
-    set -g sleep sleep .1
-    set -q CI && set sleep sleep 1
-
-    # Evil hack - override ourselves, so we only run initialization once.
-    # We could do this outside the function, but then initialization is done too early for a
-    # command that wants to set $isolated_tmux_fish_extra_args first, like
-    #   fish -C 'source tests/test_functions/isolated-tmux.fish' tests/checks/tmux-prompt.fish
     function isolated-tmux --inherit-variable tmpdir
         # tmux can't handle session sockets in paths that are too long, and macOS has a very long
         # $TMPDIR, so use a relative path - except macOS doesn't have `realpath --relative-to`...
@@ -26,6 +19,12 @@ function isolated-tmux --inherit-variable tmpdir
         rm -r $tmpdir
     end
 
+    function tmux-sleep
+        set -q CI
+        and sleep 1
+        or sleep .1
+    end
+
     set -l fish (status fish-path)
     isolated-tmux new-session -x 80 -y 10 -d $fish -C '
         # This is similar to "tests/interactive.config".
@@ -37,9 +36,12 @@ function isolated-tmux --inherit-variable tmpdir
     # Set the correct permissions for the newly created socket to allow future connections.
     # This is required at least under WSL or else each invocation will return a permissions error.
     chmod 777 .tmux-socket
-    $sleep # Let fish draw a prompt.
 
-    if set -q argv[1]
-        isolated-tmux $argv
+    # Loop a bit, until we get an initial prompt.
+    for i in seq 25
+        if string match -q '*prompt*' (isolated-tmux capture-pane -p)
+            break
+        end
+        sleep .2
     end
 end