assert: improve default error messages

This improves the error messages for:
- assert.notDeepStrictEqual
- assert.deepStrictEqual
- assert.notStrictEqual
- assert.strictEqual

Those will now always use the same error message as used in the
strict mode.

PR-URL: https://github.com/nodejs/node/pull/19467


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>