Some tests like contenders look for a lot of expected messages in the debug log at once. This is difficult to read, especially where it breaks across the terminal screen. Pretty printing the expected_msgs list fixes this.
A width of 120 instead of the usual 80 was chosen because it is very common for a hash (64 chars) to be included in expected_msgs but it can be hard to squeeze the rest of the match into the remaining 16 characters. Otherwise each expected_msgs includes a line break which just isn't necessary on most screens.
Depends on D18007