selftests/bpf: Unroll the main loop in reuseport test

Prepare for iterating over individual tests without introducing another
nested loop in the main test function.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191212102259.418536-6-jakub@cloudflare.com