Enable OpenCL tests in CI for Linux, Mac, WSL

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

This PR enables running the OpenCL tests in CI by using the POCL implementation CPU-based OpenCL. This also identified a handful of errors with the existing tests that I've fixed.

I haven't added Windows just yet, still have to see if there's a relatively simple way to include it.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[SteveBronder]

imo looks good, ty! @jgabry any comments or good to merge?

[jgabry]

I think OK to merge unless @andrjohns has anything else to push