Extend qnetworkaccesscache testing

As revealed by QTBUG-95950, some cases are not covered and need to be tested to prevent future regressions.

The various cases are:

• The cache is empty (already tested)
• The cache is not empty, and the currently inserted entry would be the new last-to-expire node in the cache (called "newest")
• The cache has at least two elements and the currently inserted entry is set to expire between the two.
• The cache is not empty, and the currently inserted entry is the new next-to-expire node in the cache (called "oldest")