Add test 'constants'

[CodeLenny]

Tests may include some constant values in the file:

const test = require("ava");

const name = "bob";

test.beforeEach("create user", t => {
  t.context.user = new User(name);
});

test("has name", t => {
  const { user } = t.context;
  t.is(user.getName(), name);
});

• Add to the "Test File Structure" list
• Add description to a section under "Test File Structure"
• See if it should be included in some of the test file examples in the README.