Add more checks in input-evdev platform probing and add umockdev acceptance tests
Now input-evdev claims to be capable of operation when it is not loaded within a nested server and is capable of opening at least one of the evdev devices it finds. mir_umock_acceptance_tests is a class of acceptance tests that require device access.
Approved by Alan Griffiths, PS Jenkins bot, Alexandros Frantzis, Brandon Schaefer.