Prefer let x+=1 for incrementing counters

The `((x++))` syntax is shorthand for `let x++`. According to `help let`:

    If the last ARG evaluates to 0, let returns 1; 0 is returned
    otherwise.

Thus the exit status of the expression `x=0; let x++` is 1, since the post-increment `++` operator evaluates to the value of the variable before incrementing.

In Bash 4, this non-zero exit status properly triggers `set -e`'s error trap, but in Bash 3 it does not. That's why the tests were passing on OS X (Bash 3) but not Linux (Bash 4).

We can work around the problem by choosing an incrementation expression that never evaluates to 0, such as `+=` or the pre-increment `++` operator. For consistency and clarity, I've changed to `x+=1` everywhere.

Ref. #25, #27

libexec/bats-exec-suite

@@ -17,7 +17,7 @@ trap "kill 0; exit 1" int
 
 count=0
 for filename in "$@"; do
-  count=$(($count + $(bats-exec-test -c "$filename")))
+  let count+="$(bats-exec-test -c "$filename")"
 done
 
 if [ -n "$count_only_flag" ]; then
@@ -35,11 +35,11 @@ for filename in "$@"; do
     while IFS= read -r line; do
       case "$line" in
       "begin "* )
-        index=$(($index + 1))
+        let index+=1
         echo "${line/ $index / $(($offset + $index)) }"
         ;;
       "ok "* | "not ok "* )
-        [ -n "$extended_syntax_flag" ] || index=$(($index + 1))
+        [ -n "$extended_syntax_flag" ] || let index+=1
         echo "${line/ $index / $(($offset + $index)) }"
         [ "${line:0:6}" != "not ok" ] || status=1
         ;;