selftest/bpf: Add test for allowed trampolines count

There's limit of 40 programs tht can be attached
to trampoline for one function. Adding test that
tries to attach that many plus one extra that needs
to fail.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200123161508.915203-4-jolsa@kernel.org