There's one set of tests that tries to sort an array of files by their
access and modification times, and that doesn't work if your filesystem
is mounted with noatime (a lot of our users do that).

This should probably be fixed upstream, and has been reported here:

https://github.com/symfony/symfony/issues/17489

--- a/Tests/Iterator/SortableIteratorTest.php
+++ b/Tests/Iterator/SortableIteratorTest.php
@@ -65,7 +65,8 @@ class SortableIteratorTest extends RealIteratorTestCase
             if ('\\' === \DIRECTORY_SEPARATOR && SortableIterator::SORT_BY_MODIFIED_TIME !== $mode) {
                 $this->markTestSkipped('Sorting by atime or ctime is not supported on Windows');
             }
-            $this->assertOrderedIteratorForGroups($expected, $iterator);
+            // $this->assertOrderedIteratorForGroups($expected, $iterator);
+            $this->markTestSkipped('Sorting by time is failure-prone on Gentoo');
         } else {
             $this->assertOrderedIterator($expected, $iterator);
         }