Complain if expect is passed multiple arguments

[gustavnikolaj]

Summary

When using unexpected it's common to import it under the name expect. Consider this case:

import expect from 'unexpected';

it('should be unexpected', () => {
  expect(true, 'to be false');
});

If a user by accident forgets to import unexpected, they will not immediately notice, as the builtin expect in jest will ignore any further arguments passed to it. Which will result in a passing test instead of the failure you would expect.

This PR adds a check to the expect method, so that it will throw an error if it is called with more than one argument.

@Munter already discussed a solution to this with @cpojer. The solution I implemented here is a bit different. Instead of using a check on arguments.length I opted to use ...rest and check that it has a length of 0. The expect method was implemented as an arrow function, and arrow functions do not expose arguments, so it was either refactoring it into a normal function or using the ...rest alternative. From my humble benchmarking I didn't observe any significant difference, so I opted for the least invasive change, that also happens to be following the Code Conventions the closest - prefering es6 syntax where possible.

To further enhance the check one could add a check for the case of expect being called with no arguments. It's less likely to cause users to shoot themselves in the feet, so I left it out.

Test plan

The test implemented covers the case that the check is implementing.

All tests in the project is passing. I ran yarn test in the root of the repo to verify that the change had no unintended side effects.

[Munter]

Discussion was in jest-core #general on Aug 9th

[cpojer]

Sounds good, and still enables us to allow more arguments in the future if we want to.

[github-actions]

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