Add unit tests for some reproducibility infrastructure

[charleskawczynski]

This PR adds some unit tests for the reproducibility infrastructure, and attempts to improve the documentation.

Thanks for suggesting this, @Sbozzolo.

One downside of this PR is that I've removed this check:

    ref_counters_main = map(read_ref_counter, ref_counter_files_main)
    if all(rc -> ref_counter_PR == rc + 1, ref_counters_main) # new reference
        @warn "`ref_counter.jl` incremented, assuming no comparable references"
        @info "Ref counters main: $ref_counters_main."
        @info "Please review output results before merging."
        return String[]
    elseif all(rc -> ref_counter_PR == rc, ref_counters_main) # unchanged reference
        @info "Ref counters main: $ref_counters_main."
        @info "Comparing results against main path:$paths"
    else
        error(
            "Unexpected reference. Please open an issue pointing to this build.",
        )
    end

But I think it no longer makes sense in the context of latest_comparable_paths returning comparable paths, because the path must contain the same ref counter as the main branch (which is now specified as a function argument).

Am I missing any edge cases that could happen?

[charleskawczynski]

I was going to add to this, but I think I'd rather merge this in smaller pieces.