Test app improvements

[avive]

This is a review of the testapp in the variable output size branch:

• [ ] Document Usage
• [ ] Uncomment bench for cpu provider
• [ ] Add provider compute engine id to bench output
• [ ] Uncomment sanity test and compare results between all providers.

[avive]

Mostly resolved in https://github.com/spacemeshos/gpu-post/pull/22 or indicated as an issue for it.