ci/flowey: split vmm_test per guest type to improve parallelism #2642
Description
As we onboard more tests, some runners take much longer to run. Today, we only create one runner per architecture, but some runners (namely the nested host runners) are slower, and take a while to run the full set of tests.
We could instead improve parallelism by splitting tests such that we run some set of them say, Windows / Linux / Other, onto their own pool and having three child jobs instead. This would increase the number of agents we need per CI run, but would vastly speed up running vmm_tests.