selftests/bpf: Use CONFIG_TEST_BPF=m instead of CONFIG_TEST_BPF=y

CONFIG_TEST_BPF can only be a module, so let's indicate it as such in
the selftests config.

Signed-off-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221205131618.1524337-4-daan.j.demeyer@gmail.com