tests/nvme: restrict tests to specific transports

tests/nvme: restrict tests to specific transports

Protect against running tests with the wrong transport type. Most tests
cannot have nvme_trtype=nvme and discovery tests expect the $trtype to
be written and verified in the .out file. Adding a couple of helpers
to restrict the transport types in tests.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>