test: refactor test-procfs-util for clarity and skip test on perm failure

test: refactor test-procfs-util for clarity and skip test on perm failure

After commit c3dead53d50e334f2d072a2248256983d6dc9f8c the test can fail
if the procfs file(s) aren't writable, because the check for permission
failure happens after a call that will never fail, since setting the
limit to the existing limit will always pass.

This also refactors the function slightly to make the test var names
clearer.