Make sure pbrt.h isn't #included before gtest.h in tests.
Make sure pbrt.h isn't #included before gtest.h in tests.

(This leads to an annoying/obscure Windows build breakage.)

mmp/pbrt-v3,shihchinw/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3