Add info about Mocha and Jest to TS docs

[vkarpov15]

What does this PR do?

Re: discussion from https://github.com/temporalio/temporal-pendulum/pull/18, I think it is better to guide devs to use any testing framework other than Jest by default. This PR makes the testing docs suggest Mocha, while still linking to the Jest sample and adding more info on 1) how to get Jest to work, and 2) supported versions of Jest. I took a look and our Jest samples don't work with Jest 26.x.

Notes to reviewers

[bergundy]

I'd keep jest as the default runner since as I mentioned in the pendulum PR and so has Loren here (with citations) it's by far the most popular test runner. Up until today we've handled all of the issues with using jest. @vkarpov15 if you've got some time maybe you can look into this one: https://github.com/temporalio/sdk-typescript/issues/753