remove some uses of $IFS

This is a step towards resolving issue #4156. It replaces uses of `$IFS`
with other solutions.

tests/interactive.config

@@ -29,13 +29,13 @@ function _echo_var --no-scope-shadowing -d '_echo_var varname [guardval]'
     case 0
         echo "\$$var has no value"
     case 1
-        set -l IFS ''
-        echo "\$$var:" (_quote $$var)
+        _quote $$var | read -l x
+        echo "\$$var: $x"
     case \*
         echo "\$$var:"
         for i in (seq (count $$var))
-            set -l IFS ''
-            echo "$i:" (_quote $$var[1][$i])
+            _quote $$var[1][$i] | read -l x
+            echo "$i: $x"
         end
     end
     if set -q argv[2]; echo "@/GUARD:$argv[2]@"; end

tests/test_util.fish

@@ -35,8 +35,9 @@ if not set -q __fish_is_running_tests
     end
 
     begin
-        set -l IFS  # clear IFS so cmd substitution doesn't split
-        cd (dirname $script); or die
+        dirname $script | read -l dir
+        cd $dir
+        or die
     end
 
     set -lx XDG_DATA_HOME ../test/data