selftests/bpf: Convert test_sysctl to prog_tests

Convert test_sysctl test to prog_tests with minimal change to the
tests themselves.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250619140603.148942-3-jmarchan@redhat.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>