Isolate smart contract tests with Truffle clean room features

In Truffle tests, we have a mysterious contract() function:

contract('my test', () => {
  if('it should ...', () => {
  });
});

It’s Truffle replacement of Mocha describe() function, but with a twist: it defines a “clean room” and it’s used to isolate tests. Very few Truffle users, but this is important to understand this: