Add Awaitility to spring-boot-starter-test

[philwebb]

Awaitility is generally useful for testing so we should consider adding it as a spring-boot-test dependency.

[vpavic]

I was just about to raise this myself when stumbled upon this issue.

Awaitility is one of testing dependencies that I regularly add to nearly all the projects, so it would be nice if spring-boot-starter-test would take care of this.

It was hinted in #18205 (comment) that this addition could be considered in the future, but it seems this topic wasn't brought up in the meanwhile.