feat/improve testing

[brianmcgee]

I've gone through root_test.go doing some clean up and refinements to how we test.

One highlight is the introduction of a better treefmt helper which allows us to do things like this:

treefmt(t,
	withArgs("elm/elm.json", "haskell/Nested/Foo.hs"),
	withNoError(t),
	withStats(t, map[stats.Type]int{
		stats.Traversed: 2,
		stats.Matched:   2,
		stats.Formatted: 0,
		stats.Changed:   0,
	}),
)

Another is using the test-fmt-append formatter more to generate changes and clarify how the caching behaviour should work in some tests.

Closes #459

[zimbatm]

amazing