Replace tests with snapshot tests

[kazk]

Replace

describe('%table-doc', () => {
  it('should render a table in HTML format', () => {
    const example = process(marked, fixture('table-doc')).html();
    expect(example).to.include('<td>id</td><td>INT</td>');
    expect(example).to.include('<td>title</td><td>VARCHAR(100)</td>');
    expect(example).to.include('<tr><td>email</td><td>VARCHAR(100)</td></tr>');
  });
});

with

describe('%table-doc', () => {
  it('should render a table in HTML format', () => {
    const example = process(marked, fixture('table-doc')).html();
    expect(format(example, { parser: 'html' })).toMatchSnapshot();
  });
});

by using Jest's expect and snapshot test. format is prettier.format.

This ensures the output doesn't change unintentionally.