Add the Test Utils Workstream and Standard

[foolip]

Closes #140.

[foolip]

@jgraham would you care to review?

[foolip]

On the naming, we could also name it after what you see in tests using this, which will be testUtils. Or possible we can name it after the Web IDL extended attribute we'll end up having to define, which might be [TestOnly] or [TestExposed]. But the risk with that is it becomes unfashionable as Web IDL keeps evolving its own naming and we want to rename it too.

I think https://testutils.spec.whatwg.org/ looks OK, but the capitalization mismatch is unfortunate.

[foolip]

We had 15 existing standards, with 5 in each of the (6, 12), (1, 7) and (2, 8) buckets. This starts a new (3, 9) bucket.

[annevk]

Need to add a calendar entry to not forgot about it when we merge this.

[annevk]

I updated this to account for the discussion. @othermaciej and @travisleithead could you please review as well?

[foolip]

Thanks @annevk, the changes look good to me!

[travisleithead]

LGTM!

[travisleithead]

Questions from: https://whatwg.org/workstream-policy#workstream-proposals

1 Proposed Workstream name:

• Provided

2 Statement of purpose, including problem to be solved, proposed solution, status of the proposed solution, and how it is consistent with the WHATWG Principles and appropriate for a Workstream.

• This is outlined in the issue; status is a proposal for one thing that is relatively mature.

3 Specific Scope of work (included and excluded).

• Scope statement is present.

4 Proposed deliverables.

• The standard section defines this (one standard)

5 Draft of the proposed Living Standard plus (optionally) a list of other proposed deliverables.

• Draft exists https://jgraham.github.io/browser-test/

6 A list of similar or related work being undertaken or proposed elsewhere and its relationship to the proposed Workstream.

• Related work was WPT, where cross-browsers tests should be provided.

7 Anticipated commencement date and timeline for work going forward.

• Not provided
  [Note remove this requirement, doesn't really make sense.]

8 Anticipated Workstream Participants.

• The editor will be the participant
  [Might also consider dropping this requirement.]

9 Description or list of those likely to implement or otherwise have an interest in the work.

• WebKit is interested, should be sufficient

10 Expectations regarding long-term maintenance of the Living Standard.

• Editor volunteering should be effectively fulfilled.

[othermaciej]

Reviewing this in light of the requirements for a Workstream Proposal: https://whatwg.org/workstream-policy#workstream-proposals

Items 1-6 and 9-10 seem effectively fulfilled by either this PR or the issue it resolves. After walking through th Workstream Proposal requirements with the SG, we agreed that 7 and 8 do not seem reasonable to ask for up front in a proposal. I agree the waive those two requirements for this PR, and we will likely file an issue to remove them.

I also approve on the substance of the PR, and I believe WebKit would be interested in implementing this spec.

[foolip]

Given that #168 has been merged I'm going to go ahead and merge this. Workstream, exist!