selftests/bpf: Allow some tests to be executed in sequence

This patch allows tests to define serial_test_name() instead of
test_name(), and this will make test_progs execute those in sequence
after all other tests finished executing concurrently.

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211006185619.364369-3-fallentree@fb.com