[Fleet] Add upgrade of Elastic stack test to e2e-testing capabilities

[EricDavisX]

First step is to have a harness here in e2e-testing to cite a 'source' or "stale" build version of stack, and then perform an install and some setup steps (some tests we already have written!) and then cite a 'upgrade-to' version to upgrade the stack to after which time we can run more tests and do specific validations.

some commentary about how upgrade tests (as seen by the test of the Agent upgrade feature) may be challenging to do during CI, so it must remain a nightly-only feature and citation for now
#652
#671

some commentary / pr on how we did the Agent upgrade feature:
#506

• just for inspiration, not that the upgrade of the Agent or the stack are terribly similar.

[EricDavisX]

@ph @mdelapenya @kevinlog just fyi. Separate effort from the custom stand-alone upgr8er tool.

[kseniia-kolpakova]

It will be covered in #1133
@EricDavisX please reopen if you have concerns