Enable parallel testing of garden-setup on multiple host-cloudproviders

[Diaphteiros]

Currently, the testmachinery can run garden-setup on one host cloudprovider. I'd like to be able to test whether creating a gardener works on multiple infrastructures. I can do that already, by starting one test for each infrastructure after the previous one is finished, but doing this sequentially takes a lot of time
Therefore I'd like to spin up a gardener on multiple infrastructures in parallel within one test.