Test cases demoing thread unsafety of prod and mock call implementations

[alshopov]

This pull request is not targeted to be merged as is

Rather it is providing test cases demoing the problem. It should be viewed together with ticket: https://github.com/yarpc/yarpc-go/issues/2357

To check:

Prod - run: go test -race

Mock - run: cd yarpctest go test -race

• [ ] Description and context for reviewers: one partner, one stranger
• [ ] Docs (package doc)

RELEASE NOTES:

[CLAassistant]

All committers have signed the CLA.