t() calls should be avoided in classes

Please review patch.

Test classes should not use t(), since a test site always uses English as language. There is no need to translate messages, except in the case the class is testing translations.

Since this issue is about t() calls, changes done to serialize() calls are off-topic.

When a string is using t()-placeholders, but the string does not need to be translated, the FormattableMarkup class should be used.

I created an issue fork, since issue forks should now be used instead of patches.

Closing this as duplicate.