6G networks are expected to meet ambitious performance parameters of coverage, data rates, latency, etc. To fulfill these objectives, the implementation of non-GEO satellite constellations is expected to improve coverage, capacity, resilience, etc. as well as the implementation of new advanced network virtualization algorithms in order to optimize network resources. However, the integration of these technologies represents new challenges, such as the execution of network slicing schemes in highly dynamic environments and network awareness requirements. In this regard, Software Defined Networking (SDN) is seen as a required 6G technology enabler in order to provide better satellite-terrestrial integration approaches and Virtual Network (VN) implementation solutions. In this paper, we present an experimental testbed for non-GEO satellite constellations integration solution and VNE algorithms implementation adapted to highly variable network conditions that builds upon SDN. A laboratory testbed has been developed and validated, consisting in SDN-based satellite-terrestrial dynamic substrate network emulated in Mininet, a Ryu SDN controller with an End-to-End (E2E) Traffic Engineering (TE) application for the VNs establishment and a Virtual Network Embedding (VNE) algorithm implemented in Matlab.