test: make RunsOnNetNextKernel() helper work with KERNEL="net-next"
test: make RunsOnNetNextKernel() helper work with KERNEL="net-next"

The NETNEXT environment variable, set to "1" or to "true", has
traditionally instructed the CI to use the net-next kernel for the
Vagrant virtual machines. For some time, the KERNEL variable has also
been assuming this role, if its value is set to "net-next" (see
test/Vagrantfile). But the relevant helper in the CI is not aware of
that, and when passing KERNEL="net-next" instead of NETNEXT="true",
ginkgo will skip tests that should run on the net-next kernel.

Let's update the relevant helper to fix this issue.

Fixes: #11061

Signed-off-by: Quentin Monnet <d6b8e48afb2534b213e391cab43016505747a234@isovalent.com>

cilium/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium