Fix running tests on a SELinux enabled system

Archivers TestMetadataChanged incorrectly clears the Extended Attributes
from the expected metadata of the temporary file. This is incorrect as on
SELinux enabled filesystem, as the kernel will automaticly add a SElinux
label. However, since ExtendedAttributes{} != ExtendedAttributes{nil} we
still need to clear them if there are no attributes found.

internal/archiver/archiver_test.go

@@ -2071,7 +2071,9 @@ func TestMetadataChanged(t *testing.T) {
 	// set some values so we can then compare the nodes
 	want.Content = node2.Content
 	want.Path = ""
-	want.ExtendedAttributes = nil
+	if len(want.ExtendedAttributes) == 0 {
+		want.ExtendedAttributes = nil
+	}
 
 	want.AccessTime = want.ModTime