[backport] Remove remaining uses of t() in test assertions

Problem/Motivation

There is no need to use t() in tests, unless we're testing translations, however in core we do not follow this consistently, which does not set a good example for new contributions.

#3153468: Strip HTML tags when using assertEquals() to compare markup exists to deal with some tricky cases in assertEquals() calls.

If we ignore those then there are just over 100 cases remaining in assertions, which seems suitable to deal with all in one go:

$ rg 'assert.*\Wt\(' core|grep -v assertEqual|wc -l
108

Steps to reproduce

Proposed resolution

Search for calls to t() in tests using the above regex and remove t() and/or replace the assertion with something more appropriate.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet