fix(model): respect discriminators with Model.validate()

[vkarpov15]

Fix #12621

Summary

Model.validate() just uses the model's schema, doesn't look for a discriminator key in obj. This PR fixes that issue. Also separated out Model.validate tests into a separate file.

Examples

[AbdelrahmanHafez]

LGTM, thanks! 👍

[IslandRhythms]

LGTM

[hasezoey]

well, after the change the error changed, but i cannot quite find the reason why it still complains about overwriting a model (all the files involved have a beforeEach to delete models)

maybe it is related that types.array.test.js sets mongoose like const mongoose = require('./common').mongoose; while the others do const mongoose = start.mongoose;

[vkarpov15]

@hasezoey afterEach() is intentional here. Not a big deal either way, afterEach() makes the tests pass so we'll use that.

[hasezoey]

afterEach() is intentional here.

what is different compared to beforeEach that would require a afterEach here unlike other test files? (just curious)

afterEach() makes the tests pass so we'll use that.

from what i can tell, it does not make it pass - that is why i had changed it to match the other test files (because of mocha's environment bleeding) ... maybe add a before to clean up before any of the tests in this file run?