test: fix use of ERRNO_IS_PRIVILEGE()

test: fix 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 read_one_line_file() which can legitimately return positive
values without errno semantics, so fix this by moving ERRNO_IS_PRIVILEGE()
invocations to the branches where the return values are known to be negative.