selftests/bpf: Add a fexit/bpf2bpf test with target bpf prog no callees

The existing fexit_bpf2bpf test covers the target progrm with callees.
This patch added a test for the target program without callees.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191205010607.177904-1-yhs@fb.com