test: Add test for new command pci_mps

Adds a test for the new pci_mps command to ensure that it can set the
Maximum Payload Size (MPS) of all devices to 256 bytes in the sandbox
environment. Enables the pci_mps command in the sandbox environment so
that this test can be run.

Signed-off-by: Stephen Carlson <stcarlso@linux.microsoft.com>