better structure filename error output

better structure filename error output

When dumping an array, PHPUnit limit the number of characters shown. That would
just hide the relevant part of the path:

1) StructureTest::testUnitTestFileNamesEndWithTest
[...]
 Array (
+    0 => '/some/path/tests/phpunit/incl...ge.php'
+    1 => '/some/path/tests/phpunit/incl...ki.php'
 )

By stripping the common path ( /some/path/tests/phpunit ), we end up with a
more useful output:

 Array (
+    0 => 'includes/RecentChange.php'
+    1 => 'includes/Wiki.php'
 )