Dev Sprint, Test Sprint - Is this scrum or waterfall?

Measure the date of dev complete and test complete difference. This difference should be not more than 2 days. If it is so, it is waterfall, and not agile.

To make this happen, you will have to make smaller and definite releases. Test team should accept builds even if there is zero functional changes (there may be just some amount of code refactoring). Releases should be regular and frequent (like once every 3 weeks). Mind you this is not end of sprint; this is middle of sprint. If you have 3 weeks for a sprint, somewhere in second week, dev team must be done with its first cut work and then it should start working with test team for acceptance, and parallely keep rest of time for subsequent releases.

sprint 1 = release 1 + release 2 activities. sprint 1 = test preparation + release 1 testing activity.
sprint 2 = release 2 + release 3 activities. sprint 2 = release 2 deployment + release 3 prep + release 3 testing.