Switch to using retest for faster development.

[KeithWM]

This PR includes only a switch from using Test to using ReTest and an arranging of the testsets into (two) different files.

The previous runtests.jl contained two testsets, MissingVectors and SentinelArrays, these now exist in separate files, missing_vectors.jl and sentinel_arrays.jl. These are both included by the new SentinelArraysTests module. The testset ChainedVector was only moved, with no modifications.