selftests/bpf: Test function_graph tracer and bpf trampoline together

Add simple test script to execute funciton graph tracer while BPF trampoline
attaches and detaches from the functions being graph traced.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191209000114.1876138-4-ast@kernel.org