vmm/lib.rs: removed mod tests
vmm/lib.rs: removed mod tests

`mod tests` from vmm/lib.rs, is not neccessary for this file,
since the units testing fit better for integration
tests, which are already covered by our current integration tests.

Signed-off-by: iulianbarbu <3d73465fd48f87db8f58a730f2d3c3f04dff43ae@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker