Refactor and enable RVec to float conversion test

Refactor and enable RVec to float conversion test

Since using CUDA directly in Google tests is not always possible,
this patch moves the cuda-specific code into separate runner file.
Also the RVec->float3 casting function is moved into new header,
thus eliminating inclusion of the cuda runtime in the tests. The
test file is renamed to reflect what is actually tested.

Fixes the post submit failure introduced in
c5c220a03663d975e31e8573c1849247ce3f8ad0, which was first avoided
by disabling the test in e91b744e8cbf2038a4dddce0609a33f396902bb5.

Fixes #3372.

Change-Id: Ie6423876e7725766109f74fdd3e76a24188b0b18