Create an overview of testing procedures

[morgo]

I was talking to a contributor today, and he asked what tests are expected for a PR.. and if there is any overview of the testing procedures. I actually don't believe there is!

By my estimate there are also at least 6 types of tests that could apply:

• unit tests
• integration tests (both in tidb repo and external)
• performance tests
• plan regression
• chaos
• scenario

[zhangyangyu]

We have a simple Write and run unit tests. It's a step-by-step guide for fresh newcomers to get started. It's would be nice if we have a richer chapter in 'Contribute to Tidb' about tests.