x86/unittests.cfg: Increase the timeout of the sieve test to 180s

In the gitlab-CI (where we are running the tests with TCG), the sieve
test sometimes takes more than 90s to finish, and thus fails due
to the 90s timeout from scripts/runtime.bash. Increase the timeout
for this test to make it always succeed.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>