Add Missing tests

[UlisesGascon]

Testing Strategy:

• Move not critical parts as external libraries with a proper testing (minimal for v2-beta1)
• Let's increase the test coverage from the action itself (minimal for v2)
• Let's cover scenarios in e2e

[KoolTheba]

Hi @UlisesGascon 👋
I've added a unit test for utility generateReportContent in the PR #32 😄

[UlisesGascon]

Thanks to @rajbos in #51 we can start moving into the E2E testing. The approach will be slightly different that the typical E2E with frameworks like Jest or Cypress but it will be good enough to grant that new collaborators can contribute and we can merge the PRs fast.

So this become again an initiative to be completed before the final 2.0.0 release 🎉