selftests/vsock: reuse logic for vsock_test through wrapper functions

selftests/vsock: reuse logic for vsock_test through wrapper functions

Add wrapper functions vm_vsock_test() and host_vsock_test() to invoke
the vsock_test binary. This encapsulates several items of repeat logic,
such as waiting for the server to reach listening state and
enabling/disabling the bash option pipefail to avoid pipe-style logging
from hiding failures.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20251108-vsock-selftests-fixes-and-improvements-v4-3-d5e8d6c87289@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>