Safely increment in a way that it works on Linux

Otherwise the CI is broken but still exits with 0 status:
https://travis-ci.org/sstephenson/bats/builds/13173769

libexec/bats-exec-test

@@ -97,7 +97,7 @@ bats_capture_stack_trace() {
       if [[ "$frame" = *"$test_pattern" ]]; then
         break
       else
-        ((index++))
+        index=$((index+1))
       fi
     done
   fi