[BUG] Unnecessary String.format() could be replaced with a simple concatenation

[lestephane]

    public ExceptionTracker stepIntoArray(final int index) {
        return this.stepInto(format("[%d]", index));
    }

could be

    public ExceptionTracker stepIntoArray(final int index) {
        return this.stepInto("[" + index + "]");
    }