selftests/bpf: Deduplicate write_sysctl() to test_progs.c

This helper is needed in multiple tests. Instead of copying it over
and over, better to deduplicate this helper to test_progs.c.

test_progs.c is chosen over testing_helpers.c because of this helper's
use of CHECK / ASSERT_*, and the CHECK was modified to use ASSERT_*
so it does not rely on a duration variable.

Suggested-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Link: https://lore.kernel.org/r/9b4fc9a27bd52f771b657b4c4090fc8d61f3a6b5.1662682323.git.zhuyifei@google.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>