test_sysctl: Add an option to prevent test skip

Tests were being skipped because the target was not present. Add a flag
that controls whether to skip a test based on the presence of the target.
Actually skip tests in the test_case function with a "return" instead of
a "continue".

Signed-off-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>