setup GPU PingTest

[jialeiwang]

This ticket is to set up PingTest for GPU computation in gpp_expected_improvement_gpu_test.hpp/cpp. To do that, we need to ensure EI and grad_EI calculation use the same sequence of random numbers.

[suntzu86]

I'm downgrading this to P4. Someone could do it if they're interested in learning but it's not important.

Reasoning: CPU EI code is fully ping tested. #219 will run the same set of random numbers thru the GPU & CPU and verify that EI/grad EI are the same. If CPU/GPU are equivalent, then GPU is implicitly ping tested already.