RSpec tests: split and sped up hg_access tests
These tests being about the security subsystem, this probably needs some hindsight.
hg_access tests in two files, then improves the performance for each.
The end figures are 2+4 minutes instead of the original 13 minutes on my development workstation, making sharding with Knapsack potentially useful again.
This MR done on the stable branch, even if we're going to submit the sharding for the default branch at first, because anything that gets those times down is welcome.