xattr key names must start with user. on linux

... or the call will just error with "operation not supported"

(cherry picked from commit 53971e8e79c515e942eb4acdb9d19d002779e5e9)

src/borg/testsuite/archiver.py

@@ -1256,14 +1256,14 @@ class ArchiverTestCase(ArchiverTestCaseBase):
             raise OSError(errno.EACCES, 'EACCES')
 
         self.create_regular_file('file')
-        xattr.setxattr('input/file', 'attribute', 'value')
+        xattr.setxattr('input/file', 'user.attribute', 'value')
         self.cmd('init', self.repository_location, '-e' 'none')
         self.cmd('create', self.repository_location + '::test', 'input')
         with changedir('output'):
             input_abspath = os.path.abspath('input/file')
             with patch.object(xattr, 'setxattr', patched_setxattr_E2BIG):
                 out = self.cmd('extract', self.repository_location + '::test', exit_code=EXIT_WARNING)
-                assert out == (input_abspath + ': Value or key of extended attribute attribute is too big for this '
+                assert out == (input_abspath + ': Value or key of extended attribute user.attribute is too big for this '
                                                'filesystem\n')
             os.remove(input_abspath)
             with patch.object(xattr, 'setxattr', patched_setxattr_ENOTSUP):
@@ -1272,7 +1272,7 @@ class ArchiverTestCase(ArchiverTestCaseBase):
             os.remove(input_abspath)
             with patch.object(xattr, 'setxattr', patched_setxattr_EACCES):
                 out = self.cmd('extract', self.repository_location + '::test', exit_code=EXIT_WARNING)
-                assert out == (input_abspath + ': Permission denied when setting extended attribute attribute\n')
+                assert out == (input_abspath + ': Permission denied when setting extended attribute user.attribute\n')
             assert os.path.isfile(input_abspath)
 
     def test_path_normalization(self):