chore: add tsconfig for tests/ directory

[achou11]

Closes #416

This doesn't include any npm script updates or a step to typecheck the tests. Mostly useful as we're writing tests for editor feedback for now.

Didn't feel inclined to include fixes to the type issues that were found as a result of this PR. Happy to try since I believe most are probably straightforward to address.

Current state of checks as of now:

Found 173 errors in 16 files.

Errors  Files
    14  tests/bitfield-rle.js:7
     4  tests/blob-store/blob-store.js:416
     5  tests/blob-store/combine-states.js:40
    12  tests/blob-store/live-download.js:19
    53  tests/core-manager.js:25
     1  tests/core-ownership.js:79
     4  tests/data-type.js:123
     4  tests/datastore.js:29
    23  tests/discovery/dns-sd.js:19
     8  tests/discovery/local-discovery.js:69
     4  tests/fastify-plugins/blobs.js:295
     7  tests/helpers/replication-state.js:3
    13  tests/invite-api.js:33
     6  tests/local-peers.js:220
     9  tests/sync/core-sync-state.js:328
     6  tests/sync/namespace-sync-state.js:111

[gmaclennan]

Looks good to me. I'd be inclined to just put // @ts-nocheck at the top of these existing files, I don't think we gain anything from jumping through hoops to make typescript happy, and it's possible that fixing some of the type issues could reduce test coverage. I can see the utility when writing new test files, and then electing to ignore errors and then add // @ts-nocheck when done.

[achou11]

I'd be inclined to just put // @ts-nocheck at the top of these existing files

Do you prefer I include that as part of this PR? Personally I'd even be fine with not doing that as long as we don't have plans to include a typechecking step for these files. I'm mostly interested in just making it easier to write the tests with the TS-powered assistance

EDIT: I think i'd actually prefer to not add the annotation at all because I still want want kind of visual feedback about potential errors that are actually type errors. i guess in theory the tests ideally break in that case but there are definitely times where passing in the wrong types for arguments is happening and not clear because of the lack of feedback for me 😄