tests/timer: Skip test_timeval_to_iso8601_overflow if we can't overflow a GTimeVal

On 32 bit systems, the size of a long might be the same as the size of an int. In that case, we won't be able to get an overflow when converting from a GTimeVal to a time_t. Skip the test for this in that case.

Closes #1509 (closed)

(cherry picked from commit f697f6aa)