Add testing (`testing` package and `neva test` command)

[emil14]

Design API proposal for testing package and neva test command.

• Research Go's testing and testify
• Check other languages
• Learn what are the most important features, what patterns arise over and over, what people complain about, what they want, how they use it
• How is unit testing different from integration and e2e, what are other kinds of testing?
• Are there any dataflow frameworks/languages/libraries that introduce testing facilities? If so how do they do it?
• How could MVP for testing look like?

Motivation: one day Neva compiler will be rewritten in Neva itself (selfhosted). We have bunch of e2e tests but a lot of stuff requires unit testing. It's very important to be able to replicate (in a idiomatic way) existing tests (where needed) before shipping new version.