cmd/dump.go uses os.Readdir to list the directory.
This is fine on its own, but TestAddRecursiveExclude in cmd/dump_test.go
depends on the order of the directory listing, which is where the issue
lays.
Directory listings using os.Readdir (lstat) don't actually guarantee an
order. They can differ due to a number of factors. Most notably the OS,
file system and settings.
As such, the test should not check the /order of the files/ added to the
archive, but instead simply check whether the archive /contains/ them.
So this is precisely what this commit does.
Note that only TestAddRecursiveExclude/File_inside_directory/No_exclude
has been observed to fail due to this, but all TestAddRecursiveExclude
subtests have been updated for consistency.
