most assets have unit tests but CI infra doesn't run them

@morgante @AdrienWalkowiak @ryanmcdowell
would love your thoughts on how we might tackle this.

## Issue
Contributors write tests that are never run and sometimes we merge changes that break tests.
we should have a part of the CI process for running tests in these assets.

## Background
Mostly the repo seems to be (mostly) standardized on
mvn test
python unittest
go test

## Future state
### Test Discovery
Ideally we'd have a test discovery / running script in the CI tool similar to check_format.sh
However this is difficult to implement because each example / tool has different dependencies and sometimes use different build tools (npm, sbt, gradle, etc.)

### New Asset Bootstrapper
If we can align on approach we can write a "new tool / new example generator script that seeds a new directory w/ a template Make / Docker file, README, etc."

### Additional CI Check on PRs
For "every dir under tools / examples has Make / Docker / OWNERS / README

## More strict contribution guidelines requirements
I'd be interested in modifying the contribution guidelines so every new asset must have a Dockerfile / Makefile to run tests. This way discovery tool could just run `make test` in each directory. This would also make reviewers job easier to pull down and run tests (which I admittedly don't usually do).

Additionally, it feels like as we try to improve the quality / maintainability of this repo we should have an OWNERS file under each tool / example. It can be difficult the track down original authors to triage issues and it'd be nice to reach out to all owners for repo wide initiatives and say...

## Messaging
What's to best way to get this message out there (and do we agree with it): "Hey we all need to migrate out of exclusions list, add docker / make / owner files by end of 2020 and upgrade to python3 or your asset will be considered unmaintained and dropped from the repo"