New test project to make specification tests easier to write for developers

[Mpdreamz]

Sunday evening having fun creating a spike of what a better test setup for us would look like.

This PR is an initial stab at porting some of the tests over without removing them from the old project.

The current test project infrastructure is still from my POC. This PR moves our tests over to F# for terser tests that we can name much better thanks to F# support for quoted identifiers:

module ``inline elements``.``anchors DEPRECATED``

open Xunit
open authoring

type ``inline anchor in the middle`` () =

    static let markdown = Setup.Markdown """
this is *regular* text and this $$$is-an-inline-anchor$$$ and this continues to be regular text
"""

    [<Fact>]
    let ``validate HTML`` () =
        markdown |> convertsToHtml """
            <p>this is <em>regular</em> text and this <a id="is-an-inline-anchor"></a> and this continues to be regular text</p>
            """
    [<Fact>]
    let ``has no errors`` () = markdown |> hasNoErrors

This allows us to name out tests MUCH better and easier:

Markdown and html are highlighted correctly (as they are in our current tests).

Further more the expected and actual html are parsed and compared at an AST level for comparison. This allows us to be less rigid with HTML formatting and format it in a way thats better for readability:

On top of that we have stronger test failure message for HTML assertions to help us diagnose much quicker since they now include a full diff:

We move to xunit v3 so we no longer have to pass ITestOutputHelper around everywhere to ensure we see errors/warnings in test explorers in the IDE.

For now we use a combination of FsUnit and Unquote for assertions but so far I am liking Unquote a lot more:

Unquote takes quoted f# expression AST executes

So e.g:

let message = errorDiagnostics.FirstOrDefault().Message
test <@ message.Contains(expected) @>

Will yield the following exception message:

message.Contains(expected)
"Substitution key {valid-key} is undefined".Contains("key {valid-key} is undefinex")
false

Since it evaluates each step of the code invocation its super clear why the test is failing.

[reakaleek]

😮