bpf: add tail call tests to test_verifier

One of the downsides of the test_bpf module was that since being
in kernel space, it couldn't test-run tail calls. Now that the
test_verifier has the ability to perform run-time tests, populate
the prog array so we actually jump into other BPF programs and
can check all corner cases. Most useful in combination with JITs.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>