Made cudftestutil header-only and removed GTest dependency

[lamarrr]

Description

This merge request follows up on #16658.
It converts cudftestutil's .cpp & .cu source files into a header source file that should be included into one translation unit at the point of consumption.
It also removes the dependency on GTest by cudftestutil. It satisfies the requirement that we only need API compatibility with the GTest API and we don't expose the gtest symbols to our consumers.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.