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-preprocess

@@ -33,7 +33,7 @@ tests=()
 index=0
 
 while IFS= read -r line; do
-  index=$(($index + 1))
+  let index+=1
   quoted_name="$(expr "$line" : ' *@test  *\([^ ].*\)  *{ *$' || true)"
 
   if [ -n "$quoted_name" ]; then