process: use templated noxfile + refactor tests

[tswast]

It'd be nice to stay consistent with the other libraries in how we perform tests.

Blocked by:

• chore: copy all samples into samples/snippets #203 - move samples to templated subdir(s)
• Python: noxfile support for extras synthtool#838 -- extras in template
• Python: noxfile support for constraints synthtool#839 -- constraints in template

[tswast]

This issue may be considered to be blocking: #336 as the SNIPPETS session is a custom session, which is not present in other libraries.

[chalmerlowe]

Closing. A number of tests and such have been relocated to more appropriate locations.