Improvement: Add more checks to longest increasing subarray test

[stefantds]

This PR proposes more tests for the "longest increasing subarray". Currently only the length of the subarray is checked. The suggested changes add more checks:

• checks if the result object is valid
• checks if the subarray is increasing

Context: while working on https://github.com/stefantds/go-epi-judge I got to look at some Java test code a bit closer. I've found some small potential improvements. I am opening a separate PR for each. Please have a look if there is something that can be improved.