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-test

@@ -97,7 +97,7 @@ bats_capture_stack_trace() {
       if [[ "$frame" = *"$test_pattern" ]]; then
         break
       else
-        ((index++))
+        let index+=1
       fi
     done
   fi
@@ -125,7 +125,7 @@ bats_print_stack_trace() {
       echo "from function \`$fn' in file $filename, line $line,"
     fi
 
-    ((index++))
+    let index+=1
   done
 }
 
@@ -197,7 +197,7 @@ bats_perform_tests() {
   status=0
   for test_name in "$@"; do
     "$0" $BATS_EXTENDED_SYNTAX "$BATS_TEST_FILENAME" "$test_name" "$test_number" || status=1
-    test_number=$(($test_number + 1))
+    let test_number+=1
   done
   exit "$status"
 }