Ensure the uniqueness of phpunit.xml coverage entries

This was already reported @ MDL-72701, don't remember if it's a regression of ot or pre-existing.

When the phpunit.xml files are generated, for coverage purposes the union of some "defaults" and the contents of the coverage.php files are joined.

But it's possible that some of the entries, say lib.php for example, are both part of the default and part of the coverage.php file.

When that happens... the phpunit.xml generated file has TWO <file>lib.php</file> entries. Same applies to directories. Both in the include and exclude sections.

While it doesn't have any negative effect, phpunit coverage seems immune to dupes... it would be nice if we can guarantee the uniqueness of the information in the phpunit.xml generated files.

Not critical at all (zero impact), but cosmetically interesting IMO. :-D

Ciao