selftests/bpf: Add test for new bpf_cpumask_first_and() kfunc

A prior patch added a new kfunc called bpf_cpumask_first_and() which
wraps cpumask_first_and(). This patch adds a selftest to validate its
behavior.

Signed-off-by: David Vernet <void@manifault.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20230610035053.117605-2-void@manifault.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>