Transition tests to use PropCheck.jl

[Seelengrab]

The current tests are an ad-hoc construction of property based testing, but they are by no means exhaustive. It would be good to utilize PropCheck.jl for this, so that we can increase the trust in the correctness of the code.

This is particularly important for testing larger field sizes, as those are currently quite untested (in spite of coverage claiming that they are).

FieldFlags.jl/test/runtests.jl

Lines 6 to 11 in 9d9591e

	#=
	Every struct created by FieldFlags is a multiple of 8 bit, so (for now) we're only going
	to test 7, 8 and 9 fields, to catch obvious off-by-one errors. That should probably
	cover multiples thereof as well, which (hopefully) is sufficient for now.
	TODO: Use PropCheck.jl to generate structs for testing instead, once PropCheck.jl has a reliable interface
	=#