test: ignore code coverage for CleanUpTest

The tests under tests/phpunit/includes/normal/CleanUpTest.php do not use
data provider. When being run for code coverage (bug 31518) they take so
long that they ends up timing out.

The whole class need to be refactored to use data providers (bug 46561).

Change-Id: I78c270e71d23ffbab00d8812ee91d380aef342c6