My character not moving anymore (in automated test) since upgrading to 5.7

You can load any UAsset file (including Pawn blueprint) with LoadObject() function.

Here an example with an USkeletalMesh but can be adapted to use APawn base class

USkeletalMesh* BatteryMesh = LoadObject<USkeletalMesh>(nullptr, TEXT("/Game/Mesh/Props/Battery/SKM_Battery.SKM_Battery"));
		if (BatteryMesh)
		{
			Battery->BatteryMesh->SetSkeletalMesh(BatteryMesh);
		}

The TEXT params is the Copy Reference action from Right Click on Content Browser

But you can’t call blueprint method if you have the intention.