Wrong sorting in drupalGetTestFiles()

In DrupalWebTestCase, drupalGetTestFiles() uses drupalCompareFiles() as callback for usort(). drupalCompareFiles($file1, $file2) does not follow the contract for sort callback functions in that drupalCompareFiles($file1, $file2) may return 1 to indicate that $file1 > $file2, while drupalCompareFiles($file2, $file1) also returns 1 for the same two files, indicating that $file2 > $file1.

Example:
$file1->name = 'foo-2.txt'; filesize($file1->filepath) = 10;
$file2->name = 'foo-1.txt'; filesize($file2->filepath) = 20;

This makes the sorted array depend on the initial sorting of the files, and that is not necessarily the same across systems. file_scan_directory() just returns the files in the order they are returned by the underlying filesystem (this was the reason for some confusing testbot results in #255551: DX: Array-itize file_scan_directory()'s parameters).