Deprecate legacy typeless addAssertion

[papandreou]

Fixes #339

I think we should get this in now, as it will make addAssertion less confusing per the linked issue. Then we can remove the support in Unexpected 12 or 13.

[papandreou]

Nice bonus info: After I changed the legacy assertions in the test suite, the check from #372 caught a handful of cases where the assertion handler mistakingly expected too many parameters. For example here: e443792?utf8=%E2%9C%93&diff=unified#diff-9d6bc70c1052e54f3f3bcbb165f036a8L258

[sunesimonsen]

Yes 👍 we should have killed it in v11, but now let's do it in v12.

[alexjeffburke]

Yes! The fact that we have this in the core test suite is as good an indication as any that the time has come. I also found (and fixed) a couple of cases of this in plugins during the v11 work. 👍