test: silence a coverity report

We check the actual contents of the file on the line after but we
might as well also check the number of bytes read here.

Found by coverity. Fixes: CID#1237521

src/test/test-fileio.c

@@ -325,7 +325,7 @@ static void test_write_string_file(void) {
 
         assert_se(write_string_file(fn, "boohoo") == 0);
 
-        assert_se(read(fd, buf, sizeof(buf)));
+        assert_se(read(fd, buf, sizeof(buf)) == 7);
         assert_se(streq(buf, "boohoo\n"));
 
         unlink(fn);