test: run mmap.rs tests with all available backends

Currently, these tests are run either with the Xen backend, or the mmap
backend, depending on which cargo features are enabled. But since the
xen feature disables mmap_unix, effectively this means if we do cargo
test --all-features, only the xen backend is tested.

Refactor our testing story to support running tests with _all_ available
backends, so that in a world where both the xen and non-xen backends can
coexist, tests will be run for both if --all-features is passed.

This won't win beauty contests, but it works for now.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat