Yes! As of May 15th, 2019, we offer reserved instance pricing in DevTest Labs:
We are excited to announce that Reserved instances discounts are now available for VMs and Azure SQL Database instances created in your Dev/Test offer subscriptions. Dev/Test offer provides you a cost-effective way to run your development and testing workloads and with the support of Reserved instances, you can enjoy additional savings and have more purchase controls for your development and test workloads.
https://azure.microsoft.com/en-us/updates/reserved-instance-pricing-in-the-dev-test-offer/