Fixed incorrect XML test suites & Usage of removed API

Change in underlying YANG Tools codec, unvereded
that there were test-suites which did string
comparison for testing of XML responses and
any change in XML serialization (valid from
XML point-of-view) triggered this tests to
fail.

Reworked offending tests to use XML W3C Document
for testing instead of string comparison and regexes.

Change-Id: Ib6e35e8d2fe79275666235370d1dda461277a59f
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>