ci: wire yarn test:typecheck into PR checks

Adds a `Typecheck tests` step to check-pr.yml that runs
`yarn test:typecheck` (tsc --noEmit -p tsconfig.test.json) in the
package/ workspace between lint and test. Tests and mock-builders are
now type-gated at PR time.

Leaves `noImplicitAny: false` in tsconfig.test.json with a TODO
comment. Flipping it exposes ~630 mechanical implicit-any errors
(destructured-parameter and `let`-variable annotations across ~60
test files). Filing as separate follow-up rather than ballooning
this PR further.

Full error count at this commit: 0.

Author: oliverlaz

.github/workflows/check-pr.yml

@@ -29,5 +29,7 @@ jobs:
         uses: ./.github/actions/install-and-build-sdk
       - name: Lint
         run: yarn lerna-workspaces run lint
+      - name: Typecheck tests
+        run: cd package && yarn test:typecheck
       - name: Test
         run: yarn test:coverage

package/tsconfig.test.json

@@ -3,6 +3,9 @@
   "compilerOptions": {
     "noUnusedLocals": false,
     "noUnusedParameters": false,
+    // TODO: flip to `true` after annotating the ~630 remaining implicit-any
+    // errors in test files (mostly destructured-parameter and `let`-variable
+    // annotations). See follow-up issue.
     "noImplicitAny": false
   },
   "include": ["./src/**/*"],