Re-enable TypeScript tests

[Methuselah96]

---

name: "Re-enable TypeScript tests"
about: Testing is important

PR Type

Does this PR add a new feature, or fix a bug?

Internal change.

Why should this PR be included?

See #3500 (comment) and following. I tried out tsd, but I did not like it as much. The expect() method syntax made it more cumbersome to assert in some scenarios. It also wasn't very well documented. I had to read the source code to figure out how to use it. So, I think sticking with typings-tester is preferable for now.

Note, I also needed to add an import in types\store.ts in order to get the global Symbol.observable definition working in the type outputs.

Checklist

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Is there an existing issue for this PR?
  • No, this is an internal change.
• Have the files been linted and formatted?
• Have the docs been updated to match the changes in the PR?
• Have the tests been updated to match the changes in the PR?
• Have you run the tests locally to confirm they pass?

Bug Fixes

What is the current behavior, and the steps to reproduce the issue?

Types can break.

What is the expected behavior?

The ability to test types.

How does this PR fix the problem?

Re-enables types testing.

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[timdorr]

Ah, we had somehow missed this. Thanks for catching and fixing it!