test-char: abort on serial test error

We are having issues debugging and bisecting this issue that happen
mostly on patchew. Let's make it abort where it failed to gather some
new informations.

Suggested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

tests/test-char.c

@@ -1200,7 +1200,7 @@ static void char_serial_test(void)
 
     /* test tty alias */
     qemu_opt_set(opts, "backend", "tty", &error_abort);
-    chr = qemu_chr_new_from_opts(opts, NULL, NULL);
+    chr = qemu_chr_new_from_opts(opts, NULL, &error_abort);
     g_assert_nonnull(chr);
     object_unparent(OBJECT(chr));