tests/unit: fix a -Wformat-truncation warning

tests/unit: fix a -Wformat-truncation warning

../tests/test-qobject-input-visitor.c: In function ‘test_visitor_in_list’:
../tests/test-qobject-input-visitor.c:454:49: warning: ‘%d’ directive output may be truncated writing between 1 and 10 bytes into a region of size 6 [-Wformat-truncation=]
  454 |         snprintf(string, sizeof(string), "string%d", i);
      |                                                 ^~
../tests/test-qobject-input-visitor.c:454:42: note: directive argument in the range [0, 2147483606]
  454 |         snprintf(string, sizeof(string), "string%d", i);
      |                                          ^~~~~~~~~~
../tests/test-qobject-input-visitor.c:454:9: note: ‘snprintf’ output between 8 and 17 bytes into a destination of size 12
  454 |         snprintf(string, sizeof(string), "string%d", i);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Rather than trying to be clever, since this is called 3 times during
tests, let's simply use g_strdup_printf().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20220810121513.1356081-1-marcandre.lureau@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: fixed commit message typos]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>