tools/testing/selftests/sysctl/sysctl.sh: allow graceful use on older kernels

On old kernels older new test knobs implemented on the test_sysctl
module may not be available.  This is expected, and the selftests test
scripts should be able to run without failures on older kernels.

Generalize a solution so that we test for each required test target file
for each test by requiring each test description to annotate their
respective test target file.  If the target file does not exist, we skip
the test gracefully.

Link: http://lkml.kernel.org/r/20190320222831.8243-5-mcgrof@kernel.org


Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Eric Sandeen <sandeen@redhat.com>
Cc: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>