Split up test_reference into ~3 smaller functions

[oxinabox]

test_reference is like 50 lines long.

ReferenceTests.jl/src/test_reference.jl

Lines 93 to 150 in f26bec0

	function test_reference(
	file::File{F},
	raw_actual::T,
	equiv=nothing,
	rendermode=nothing;
	kw...) where {F <: DataFormat, T}
	path = file.filename
	dir, filename = splitdir(path)
	actual = _convert(F, raw_actual; kw...)
	# infer the default rendermode here
	# since `nothing` is always passed to this method from
	# test_reference(filename::AbstractString, raw_actual; kw...)
	if rendermode === nothing
	rendermode = default_rendermode(F, actual)
	end
	# preprocessing when reference file doesn't exists
	if !isfile(path)
	@info("Reference file for \"$filename\" does not exist. It will be created")
	# TODO: move encoding out from render
	render(rendermode, actual)
	mkpath(dir)
	savefile(file, actual)
	@info("Please run the tests again for any changes to take effect")
	return nothing # skip current test case
	end
	# file exists
	reference = loadfile(typeof(actual), file)
	if equiv === nothing
	# generally, `reference` and `actual` are of the same type after preprocessing
	equiv = default_equality(reference, actual)
	end
	if equiv(reference, actual)
	@test true # to increase test counter if reached
	else
	# post-processing when test fails
	println("Test for \"$filename\" failed.")
	render(rendermode, reference, actual)
	if !isinteractive()
	error("You need to run the tests interactively with 'include(\"test/runtests.jl\")' to update reference images")
	end
	if !input_bool("Replace reference with actual result (path: $path)?")
	@test false
	else
	savefile(file, actual)
	@info("Please run the tests again for any changes to take effect")
	end
	end

And it is even longer when i put in the stuff for #79

I think we should have it more like

function test_reference(
    file::File{F},
    raw_actual::T,
    equiv=nothing,
    rendermode=nothing;
    kw...) where {F <: DataFormat, T}

    actual = ...
    rendermode = ...

    if !isfile(path)
        handle_nonexisting_reference(...)
    else
        test_against_reference(...)
    end
end

splitting the two main branchs out.
Possible workout out the equiv should be done in the main function, then all the modal stuff is done there.