test-fileio: fix confusing log output

Previously we'd pass all return values of read_virtual_file() to
log_info_errno() as error, but that makes no sense, given that we
sometimes return positive one with means "not truncated" but we'd show
as "Permission denied. Let's fix this, and log differently for sucess
and error.

src/test/test-fileio.c

@@ -1034,8 +1034,11 @@ static void test_read_virtual_file(size_t max_size) {
                 size_t size = 0;
 
                 r = read_virtual_file(filename, max_size, &buf, &size);
-                log_info_errno(r, "read_virtual_file(\"%s\", %zu): %m (%zu bytes)", filename, max_size, size);
-                assert_se(r == 0 || ERRNO_IS_PRIVILEGE(r) || r == -ENOENT);
+                if (r < 0) {
+                        log_info_errno(r, "read_virtual_file(\"%s\", %zu): %m", filename, max_size);
+                        assert_se(ERRNO_IS_PRIVILEGE(r) || r == -ENOENT);
+                } else
+                        log_info("read_virtual_file(\"%s\", %zu): %s (%zu bytes)", filename, max_size, r ? "non-truncated" : "truncated", size);
         }
 }