That's the same thing as Item2158, only that the cure needs to be applied to most test suites:

Many unit tests create a TWiki->new() during their set_up routine, i.e. repeatedly for each test. But we know since Item2158 that TWiki objects contain circular references, so they need to be properly destroyed.

The easiest way would be to call $twiki->finish() during the tear_down() routine in all those packages which create the object during set_up().