The issue has been open since 2009. It has consensus and the approval of SimpleTest maintainers, and is already part of our documented standard: http://drupal.org/simpletest-tutorial-drupal7#t
Details
The $message parameter of SimpleTest assertions (e.g., DrupalTestCase::assertTrue()) is a string displayed only in the administrative UI or on the commandline following test runs, and on testbot.
This parameter is never translated, so using t() on it is needless overhead.
We have other instances in core where string function parameters should not be wrapped in t(), e.g. watchdog().
When the assertion message is a plain string, t() should simply be omitted.
When the assertion message contains placeholders for variables, format_string() should be used instead.
Note: Whenever you pass a message into a SimpleTest assertion, do NOT translate the message using t(). (To format a string with variables, use format_string() instead.)
We should also add a regex in PHPCS to be catching these (or in CSpell if not possible PHPCS).
This has been on my list for a while, but never filed an issue for it before.
See https://www.drupal.org/project/drupal/issues/500866
https://www.drupal.org/docs/7/testing/assertions (which is what we are following in Backdrop as well AFAIK) mentions that already:
We should also add a regex in PHPCS to be catching these (or in CSpell if not possible PHPCS).