Add 'src' folder to Jest roots

[itsananderson]

I was poking at the Jest CLI trying to get it to just run related tests when I change source files, but it was failing to find related tests.

According to this, the reason Jest can't find related tests is because the src folder isn't in the roots list in the Jest config.

This PR updates the roots config to include the src folder.

Before:

> yarn jest --findRelatedTests src/api-review-state.ts
yarn run v1.22.22
$ C:\Users\itsananderson\src\cation\node_modules\.bin\jest --findRelatedTests src/api-review-state.ts
No tests found, exiting with code 1
Run with `--passWithNoTests` to exit with code 0
In C:\Users\itsananderson\src\cation
  47 files checked.
  testMatch:  - 0 matches
  testPathIgnorePatterns: spec\\utils.ts - 46 matches
  testRegex: (\\spec\\.*|(\.|\\)(test|spec))\.tsx?$ - 6 matches
Pattern: src\\api-review-state.ts - 0 matches
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

After:

> yarn jest --findRelatedTests src/api-review-state.ts

... snip ...

Test Suites: 2 passed, 2 total
Tests:       35 passed, 35 total
Snapshots:   0 total
Time:        19.175 s, estimated 20 s
Ran all test suites related to files matching /src\\api-review-state.ts/i.
Done in 19.82s.