Revise how to run the functional tests using Open Liberty dev mode

Follow up on https://github.com/OpenLiberty/sample-getting-started/pull/69#pullrequestreview-1032096915

Replace the failsafe plugin in favor of Open Liberty dev mode to run the functional tests. Ideally, this work is to add more documentation in the `README.md` on how to run the tests through Open Liberty dev mode.

Currently the tests are documented using `maven-failsafe-plugin`