Adjust plan

Per discussion[^1], the adjustment is needed due to PR#179 removing
some redundant tests, while forgetting to adjust the plan.

#179 (comment)

S03-operators/mix.t

@@ -1,7 +1,7 @@
 use v6;
 use Test;
 
-plan 128;
+plan 126;
 
 sub showset($b) { $b.keys.sort.join(' ') }