selftests/bpf: Normalize selftest entry points

Ensure that all test entry points are global void functions with no
input arguments. Mark few subtest entry points as static.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211022223228.99920-2-andrii@kernel.org