[Fleet] Create an e2e test for the Fleet saved objects storage in ES feature work done in 7.12

[EricDavisX]

The 8.0 / 7.12 feature test ticket is here: https://github.com/elastic/kibana/issues/84967

I will post specific test steps in the test issue above and then translate them here as possible into API (only) steps which we can automate into a scenario.