This commit fixes a warning from tar that the archive cannot contain
itself. It also makes any tar output a warning (maybe even an error?)
as we do not expect anything from the tar command. The test is updated
to also check this.
The `assert.Equal()` expects that the "expected" value is put
first. Which is not what I'm used to. It's also slightly inconsistent
because `assert.EqualError()` expects the "actual" err first and
then the expected string. But this commit is not about ranting :)
This commit fixes the order in the tests assert.Equal() so that
mismatches actually are displayed correctly.
