Documentation for Migrating LB3 Tests Mounted on an LB4 App

[nabdelgadir]

Followup for #5251. Related issue: #3978.

Add documentation instructing users how to migrate their LB3 tests after they mount their LB3 app on an LB4 project.

Acceptance criteria

• Create a section in lb3-application's README for migrating tests
  • Describe what needs to be done before running the tests (e.g. adding lb3 directory to npm test command)
  • Describe how to migrate acceptance tests
  • Describe how to migrate integration tests

[jannyHou]

Closing after #5403 merged.