Add harness to npm package

[Christian24]

Description

Hi,
I am currently working on a Lit integration for Tanstack Form. I am currently in the process of writing tests and sadly there is no good framework for writing web component tests.

I have opened a couple testing library PRs but they haven't been merged in ages: testing-library/user-event#1033

Hence, I am looking for a different way to simulate input in my tests. Since I am using Material Web for components I thought maybe I could use the Harness: https://github.com/material-components/material-web/blob/main/testing/harness.ts Sadly, it seems harness is not part of the npm package however.

Is there a reason for this? Angular Material does provide component harnesses and I would welcome if Material web did too.

[asyncLiz]

Uh oh! This looks like an oversight.

We do ship per-component harnesses (like @material/web/button/harness.js), but it looks like we're excluding the testing folder which it needs 😬

[Christian24]

Oh! I opened a PR that should fix this then @asyncLiz :)