Make random test more fault tolerant

It's valid to get one or two duplicates for multiple
calls to random. Update test to take it into account!

Author: mgreter

spec/libsass-closed-issues/issue_657/default/input.scss

@@ -1,22 +1,22 @@
 $values: ();
 
 foo {
+    $duplicates: 0;
     $num: random();
     $is-number: type-of($num) == number;
     $is-within-range: $num >= 0 and $num < 1;
-    $is-random: index($values, $num) == null;
     $values: append($values, $num);
 
     @for $i from 1 through 1000 {
       $num: random();
       $is-number: $is-number and type-of($num) == number;
       $is-within-range: $is-within-range and $num >= 0 and $num < 1;
-      $is-random: $is-random and index($values, $num) == null;
+      @if (index($values, $num) != null) { $duplicates: $duplicates + 1; }
       $values: append($values, $num);
     }
 
     is-defined: $num != "random()";
     is-number: $is-number;
     is-within-range: $is-within-range;
-    is-random: $is-random;
+    is-random: $duplicates < 25;
 }