selftests/rdma: explicitly skip tests when required modules are missing

selftests/rdma: explicitly skip tests when required modules are missing

Currently, the rdma rxe selftests fail with an exit code of 1 when
required kernel modules are not present. This causes spurious failures
in environments where these modules might not be compiled or available.

Include the standard kselftest 'ktap_helpers.sh' and replace the
hardcoded error exits with '$KSFT_SKIP'. This ensures the tests are
properly marked as skipped rather than failed.

Fixes: e01027cab38a ("RDMA/rxe: Add testcase for net namespace rxe")
Signed-off-by: Yi Lai <yi1.lai@intel.com>
Link: https://patch.msgid.link/20260507125106.3114167-1-yi1.lai@intel.com
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Leon Romanovsky <leon@kernel.org>