blktests: _nvme_connect_subsys() improvement for RT kernel

[Shinichiro Kawasaki]

FYI, Yi Zhang ran blktests on RT kernel and observed that several test cases in
nvme group failed. To avoid the failures, Yi worked on the blktests script
improvement which modified _nvme_connect_subsys() and some of nvme test cases.
In case you are interested in the work, please take a look in the GitHub PR [1].

[1] https://github.com/linux-blktests/blktests/pull/194