test/nvmf: Add 'if' condition for nvmf cleanup

Introduce SPDK_TEST_SKIP_NVMF_KERNEL_TESTS flag to optionally skip
the nvmfcleanup step. When enabled, this avoids unnecessary kernel
module operations and reduces nvmf test duration to 40%.

Recommended for constrained setups (e.g., containers with preloaded
NVMe kernel modules).

By default, the flag is disabled and behavior remains unchanged.

Change-Id: I41b6ccc081ec0f8389d7613a7373b3f3f00f9f79
Signed-off-by: Przemyslaw Wielgo <przemyslaw.wielgo@dell.com>
Reviewed-on: https://review.spdk.io/c/spdk/spdk/+/26733


Tested-by: SPDK Automated Test System <spdkbot@gmail.com>
Reviewed-by: Tomasz Zawadzki <tomasz@tzawadzki.com>
Reviewed-by: Michal Berger <michal.berger@nutanix.com>
Reviewed-by: Jim Harris <jim.harris@nvidia.com>