chore: enable type checking for all tests (#669)

_This only affects type-checking in tests and should have no user
impact._

Before this change, we weren't type-checking the `tests/` folder. Now we
are!

Prerequisites that made this change possible (list might be incomplete):

- ebcd4d2
- df66656
- 91d8c9c
- eca4741
- f64ca17
- 0d3d8d9
- 57cfe72
- 95a1fa9
- b7226aa
- 0121b2d
- 079c7e5
- d7b0268
- 3bb941f
- 2fe45ee
- bf56451
- 23416ab

tsconfig.json

@@ -26,6 +26,6 @@
       "*": ["./types", "./node_modules/@digidem/types/vendor/*/index.d.ts"]
     }
   },
-  "include": ["src/**/*", "types/**/*.d.ts", "test-e2e/**/*"],
+  "include": ["src/**/*", "types/**/*.d.ts", "tests/**/*", "test-e2e/**/*"],
   "exclude": ["node_modules", "src/generated/*.js"]
 }