tests/unit: Bump test-aio-multithread test timeout to 2 minutes

When running the tests in slow mode on a very loaded system and with
--enable-debug, the test-aio-multithread can take longer than 1 minute.
Bump the timeout to two minutes to make sure that it also passes in
such situations.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20231215070357.10888-14-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>