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

tools/testing/selftests/bpf/config

@@ -71,7 +71,7 @@ CONFIG_NF_NAT=y
 CONFIG_RC_CORE=y
 CONFIG_SECURITY=y
 CONFIG_SECURITYFS=y
-CONFIG_TEST_BPF=y
+CONFIG_TEST_BPF=m
 CONFIG_USERFAULTFD=y
 CONFIG_VXLAN=y
 CONFIG_XDP_SOCKETS=y