Implement preloading

[MattWindsor91]

Quote Alglave et al:

'If the (default) preload mode is enabled, a preliminary loop of size s reads a random subset of the memory locations accessed by Pk, also leading to a greater outcome variability.'

I'm not entirely sure when this loop occurs without reading the generated litmus source (which I'm trying to avoid as much as possible to avoid it colouring my implementation!), but I guess it's probably just once per thread, done as if it's just a normal test, but with a different test body.