Utility to prevent using toEqual on entities

[stephenh]

When Jest diffs entities for like expect(result).toEqual(someEntity), it deep crawls into each entity and ends up diffing the entire EntityManager / connection pool / etc.

Besides just being slow, if the assertions ever fail, the diff output is huge.

We've added toMatchEntity to work around the inability to custom toEquals diff output, but it's easy to forget to use it.

We could probably add a custom equality matcher, see here:

https://github.com/jestjs/jest/blob/9113f44b459f96db31e5ad3811d4d8b07f79e54f/packages/expect-utils/src/jasmineUtils.ts#L79

That doesn't actually provide custom equality, but instead just tells the user to use toMatchEntity instead.