test: cleanup use of ERRNO_IS_PRIVILEGE()

test: cleanup use of ERRNO_IS_PRIVILEGE()

Given that ERRNO_IS_PRIVILEGE() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the arguments passed to ERRNO_IS_PRIVILEGE() are the values
returned by procfs_get_pid_max() and procfs_get_threads_max() which are
not expected to return any positive values, but let's be consistent
anyway and move ERRNO_IS_PRIVILEGE() invocations to the branches where
the return values are known to be negative.