fix probe tests to work in a VM using mount_check

If mount_check is true (ie an SAIO with "real "devices, not loopback),
then the servers will correctly return 507 when given a nonsense path.
The first element is treated as a drive path, and that path isn't
mounted. This patch adds 507 as a valid status response to the server
check.

Change-Id: I1d1bb0ab78fd9ea17323635da7e686182fbdbf13
Merge "fix probe tests to work in a VM using mount_check"