Replace assertRevert with expectRevert

[andrewgordstewart]

Related: magmo/apps#162

The assertRevert function uses chai assertions, and does not block test success until the promise resolves, forcing the user to add a delay after the assertion.

The following expectRevert blocks jest tests until the promise returned by fn resolves, which is far superior:

async function expectRevert(fn, msg) {
  expect.assertions(1);
  await expect(fn()).rejects.toThrowError(msg);
}