Modernize AssertContentTrait

Problem/Motivation

AssertContentTrait adds several helpers for tests, but its API (with custom message and group) is much closer to SImpletest than phpunit. We can bring it up to date a bit. Pointed out by @xjm on #3142749: AssertLegacyTrait::assertPattern() calls in functional tests still have a message passed in.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet