Add test script to web

[Tobbe]

This adds a test script to the web project, to include it in the main test run executed by GitHub's actions.

So now this is part of the action output:

@redwoodjs/web: $ jest --colors
@redwoodjs/router:  PASS  src/__tests__/util.test.js
@redwoodjs/router:  PASS  src/__tests__/links.test.js
@redwoodjs/web:  PASS  src/flash/__tests__/FlashContext.test.js
@redwoodjs/router:  PASS  src/__tests__/location.test.js
@redwoodjs/router: Test Suites: 3 passed, 3 total
@redwoodjs/router: Tests:       43 passed, 43 total
@redwoodjs/router: Snapshots:   0 total
@redwoodjs/router: Time:        4.428s
@redwoodjs/router: Ran all test suites matching /src/i.
@redwoodjs/web:  PASS  src/flash/__tests__/flash.test.js
@redwoodjs/web: Test Suites: 2 passed, 2 total
@redwoodjs/web: Tests:       7 passed, 7 total
@redwoodjs/web: Snapshots:   0 total
@redwoodjs/web: Time:        3.212s
@redwoodjs/web: Ran all test suites.

All those @redwoodjs/web lines are new