fixture: Add missing trailing NUL in security.selinux xattr

Because we were only doing bare-user checkouts in the test
suite (and not trying to physically materialize the xattrs)
and because ostree today doesn't validate xattrs, this
problem was silently ignored.  But I was looking at
changing the tests to do a checkout, and this caused confusing
failures.

Add the missing trailing `NUL`.  Also while we're here,
split the xattr function in two; one which returns the
Rust-native value and a wrapper which converts to GVariant.

This will be useful later.

lib/src/fixture.rs

@@ -229,8 +229,12 @@ impl SeLabel {
         }
     }
 
+    pub fn xattrs(&self) -> Vec<(&[u8], &[u8])> {
+        vec![(b"security.selinux\0", self.to_str().as_bytes())]
+    }
+
     pub fn new_xattrs(&self) -> glib::Variant {
-        vec![("security.selinux".as_bytes(), self.to_str().as_bytes())].to_variant()
+        self.xattrs().to_variant()
     }
 }