selftests/bpf: Use "is not set" instead of "=n"

"=n" is not valid kconfig syntax. Use "is not set" instead to indicate
the option should be disabled.

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/20221205131618.1524337-3-daan.j.demeyer@gmail.com