Remove uses of FormattableMarkup in kernel test assertions

Problem/Motivation

There is no need to use FormattableMarkup to produce message strings in PHPUnit assertions; it is cleaner and easier in almost all cases to use simple string interpolation instead.

Steps to reproduce

Proposed resolution

Replace all uses of FormattableMarkup in kernel test assertions with either plain string interpolation, or remove the assertion message entirely when the assertion is obvious and the message does not add any context.

Remaining tasks

grep FormattableMarkup -r -l | grep Kernel

User interface changes

API changes

Data model changes

Release notes snippet