IVGCVSW-3547 Use ExecuteNetwork to run a dynamic backend end to end test

IVGCVSW-3547 Use ExecuteNetwork to run a dynamic backend end to end test

 * Added path override for dynamic backend loading
 * Do not default to CpuRef, as there could be dynamic backends loaded at
   runtime
 * Do not check right away whether the backends are correct, as more of
   them can be loaded at runtime as dynamic backends

Change-Id: If23f79aa1480b8dfce57e49b1746c23b6b9e6f82
Signed-off-by: Matteo Martincigh <matteo.martincigh@arm.com>