Promised-based testing using console / error / info in ~60 LoC complete with suites, custom reporters, summaries and a few assertions via Error.

duct.js

const done = () => {}
const indent = (depth, info) => [...Array(depth)].reduce(p => `${p}  `, '') + info;
const reporter = {
  fail: ({ depth, message, name }) => {
    console.info(indent(depth, `✗ ${name}`));
    console.error(indent(depth + 1, message));
  },
  pass: ({ depth, name }) => console.info(indent(depth, `✓ ${name}`)),
  suite: ({ depth, name }) => console.info(indent(depth, name)),
  test: () => {}
};

let deferred = Promise.resolve();
let depth = 0;

function test (name, func, opts = { done, reporter }) {
  const tests = [];
  deferred
    .then(() => reporter.suite({ depth, name }))
    .then(() => ++depth);
  func(function (name, ...results) {
    deferred
      .then(() => tests.push({ errors, depth, names: suites.concat(name) }));
    deferred = results.reduce((a, b) => a
      .then(() => reporter.test({ depth, name }))
      .catch(e => {
        tests[tests.length - 1].errors.push(e.message);
        reporter.fail({ depth, message: e.message, name });
      })
      .then(b)
      .then(r => reporter.pass({ depth, name })), deferred);
    deferred
      .then(() => --depth);
  });
  deferred
    .then(() => done(tests));
}

function str (v) {
  return JSON.stringify(v);
}

function toss (msg, info) {
  throw new Error(`${msg}${info ? `: ${info}` : ''}`);
}

function assert(v, info) {
  if (!v) {
    toss('assertion failed', info);
  }
}

function eq (v1, v2, info) {
  if (v1 !== v2) {
    toss(`expected ${str(v1)} to *not* equal ${str(v2)}`, info);
  }
}

function ne (v1, v2, info) {
  if (v1 === v2) {
    toss(`expected ${str(v1)} to *not* equal ${str(v2)}`, info);
  }
}