injection_points: Add stats for point caching and loading

injection_points: Add stats for point caching and loading

This adds two counters to the fixed-numbered stats of injection points
to track the number of times injection points have been cached and
loaded from the cache, as of the additions coming from a0a5869a8598 and
4b211003ecc2.

These should have been part of f68cd847fa40, but I have lacked time and
energy back then, and it did not prevent the code to be a useful
template.

While on it, this commit simplifies the description of a few tests while
adding coverage for the new stats data.

Author: Yogesh Sharma
Discussion: https://postgr.es/m/3a6977f7-54ab-43ce-8806-11d5e15526a2@catprosystems.com