Fix flaky GPUNEP MD tests

The shared custom_parameters fixture ran a nvt_ber MD trajectory without a velocity command, so GPUMD initialized velocities randomly at 300 K on every run, making the final energy/stress/forces non-deterministic. Two tests asserted exact hardcoded values against this random trajectory and failed intermittently.

Add velocity 300 seed 12345 to the fixture to pin the trajectory, and update the hardcoded expected energy, stress, and forces to match the now-deterministic output. Verified by rerunning both tests repeatedly with no flakiness.