Tests: Use arbitrary OpCodes when testing Queue
Tests: Use arbitrary OpCodes when testing Queue

Before, `OpClusterQuery` was the only OpCode used.
This change uses `OpCode`'s `Arbitrary` instance, which revealed
that this instance was too loose, generating invalid String fields
(fixed in the preceding commit).

Signed-off-by: Niklas Hambuechen <98215d3a72cb13cc9c9956528e50119ca7d1fb8b@google.com>
Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>

grnet/snf-ganeti,yiannist/ganeti,ganeti/ganeti,andir/ganeti,mbakke/ganeti,bitemyapp/ganeti,dimara/ganeti,apyrgio/ganeti,yiannist/ganeti,onponomarev/ganeti,andir/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,andir/ganeti,grnet/snf-ganeti,mbakke/ganeti,leshchevds/ganeti,bitemyapp/ganeti,onponomarev/ganeti,leshchevds/ganeti,ganeti/ganeti,mbakke/ganeti,dimara/ganeti,leshchevds/ganeti