Remove uses of t() in assert[No]FieldBy*() calls

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.

In #3133726: [meta] Remove usage of t() in tests not testing translation we identified there are severals of calls to t() in calls to assertFieldByName(), assertFieldByXpath() and assertNoFieldByXPath() and that removing all these in one go seems to be a suitable way of attacking this problem.

Proposed resolution

Identify and remove all calls to t() wrapped in calls to assertFieldByName(), assertFieldByXpath() and assertNoFieldByXPath(), except those used by translation-related code (if any).

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet