selftests/net: Use public NetDrvContEnv API in nk_qlease fixtures

selftests/net: Use public NetDrvContEnv API in nk_qlease fixtures

Expose the netkit host ifname as a public attribute nk_host_ifname
(symmetric with the already-public nk_guest_ifname), rename _attach_bpf
to a public attach_bpf, and add a public detach_bpf helper that
encapsulates the tc-filter teardown bookkeeping. Switch the fixtures
to this public API. No functional change and keeps pylint happy.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260614102607.863838-3-daniel@iogearbox.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>