The ATF header isn't really supposed to be used this way: it's executed even when the test itself is not, so kyua will run this code when interrogating tests for their metadata. See atf-test-case.4. Instead of having a header for each test which wants to know the page size, perhaps have

static size_t
pagesize(void)
{
    static int ps = 0;

    if (ps == 0)
         ps = getpagesize();
    return (ps);
}

and make each test call it where it uses page_size now?