If a test is marked as a flake then it will be tried five times before
giving up.
Works around the problem of needing to babysit CI once a PR is pushed.
As long as we have all the flake tests marked.
cmake will bake in the environment variable in to the build scripts.
Instead have the guest_test_runner fetch it at runtime.
This means if you forget to set ROOTFS prior to running cmake, you can
now set it afterwards and rerun with just ctest instead of a cmake
dance.
Fixes#315
Ryan Houdek