test-terminal-util: fix sigsegv when compiled without asserts

I couldn't figure out what is going on here, because LTO inlines everything and
then the backtrace reported a different spot. But when compiled with NDEBUG but
no LTO, it's fairly obvious ;)

C.f. #12008.

src/test/test-terminal-util.c

@@ -40,7 +40,7 @@ static void test_read_one_char(void) {
 
         log_info("/* %s */", __func__);
 
-        assert(fmkostemp_safe(name, "r+", &file) == 0);
+        assert_se(fmkostemp_safe(name, "r+", &file) == 0);
 
         assert_se(fputs("c\n", file) >= 0);
         rewind(file);