* tests/dircolors/simple (a): Don't fail with an unexpected diagnostic

when the shell variable, SHELL, is not set.
Trigger the failure with "(unset SHELL; make check TESTS=simple)".
Reported by Sven Joachim in <http://bugs.debian.org/355368>.

tests/dircolors/simple

@@ -38,7 +38,7 @@ use strict;
 
 my @Tests =
     (
-     ['a', {IN => {k => "exec\n"}},
+     ['a', '-b', {IN => {k => "exec\n"}},
       {ERR => "dircolors: k:1: invalid line;  missing second token\n"},
       {EXIT => 1}],
      ['quote', '-b', {IN => "exec 'echo Hello;:'\n"},