tests: Move control sequences filtering to fish directly

This was an sh-script that just invoked fish again.

I can see how we could implement it in another language to avoid the
fish under test corrupting the results, but it literally invoked the
fish under test anyway.

tests/filter-control-sequences.fish

@@ -0,0 +1,17 @@
+#!/usr/bin/fish
+# Remove the sorts of escape sequences interactive fish prints.
+
+# First the enable sequences
+set -l escapes "\e\[\?2004h"\
+"\e\[>4;1m"\
+"\e\[>5u"\
+"\e="\
+# or
+"|"\
+# the disable sequences
+"\e\[\?2004l"\
+"\e\[>4;0m"\
+"\e\[<1u"\
+"\e>"
+
+cat | string replace -ra -- $escapes ''

tests/filter-control-sequences.sh

@@ -1,14 +0,0 @@
-#!/bin/sh
-
-escape=$(printf '\033')
-
-"$1" -c 'cat | string replace -ra $argv[1] ""' -- "\
-""$escape\[\?2004h""\
-""$escape\[>4;1m""\
-""$escape\[>5u""\
-""$escape=""\
-""|""\
-""$escape\[\?2004l""\
-""$escape\[>4;0m""\
-""$escape\[<1u""\
-""$escape>"

tests/test.fish

@@ -42,7 +42,7 @@ if set -q files_to_test[1]
         --progress $force_color \
         -s fish=../test/root/bin/fish \
         -s fish_test_helper=../test/root/bin/fish_test_helper \
-        -s filter-control-sequences='../tests/filter-control-sequences.sh ../test/root/bin/fish' \
+        -s filter-control-sequences='../test/root/bin/fish ../tests/filter-control-sequences.fish' \
         $files_to_test
 
     set -l littlecheck_status $status