Add API test project.

[Arkatufus]

Add API test using ApiApproval

[Aaronontheweb]

@Arkatufus I'd mark these tests as skippable on Linux - like how we do here for the Hocon.Configuration tests:

HOCON/build.fsx

Lines 95 to 99 in cbaf428

	let projects =
	match (isWindows) with
	| true -> !! "./src/**/*.Tests.csproj"
	| _ -> !! "./src/**/*.Tests.csproj"
	-- "./src/**/Hocon.Configuration.Tests.csproj" // Mono barfs on this spec

Some of the plumbing that goes on behind the scenes doesn't play nicely with Linux, for looking up diffing tools I believe.

[Arkatufus]

OK, I've removed Hocon.API.Tests from the build script.