In the test suite's checking of multi-line property values, fix a line-ending

In the test suite's checking of multi-line property values, fix a line-ending
quirk and make the parsing a bit more robust and simpler.

* subversion/tests/cmdline/merge_tests.py
  (avoid_repeated_merge_on_subtree_with_merge_info,
   merge_old_and_new_revs_from_renamed_dir,
   merge_source_normalization_and_subtree_merges,
   dont_add_mergeinfo_from_own_history): Remove the spurious newline from
    the end of the expected mergeinfo value.

* subversion/tests/cmdline/svntest/tree.py
  (get_props): Don't add an extra newline to a multi-line property value.
    Be more careful in matching a property name line. When stripping the
    line-ending characters, avoid accidentally stripping other white space
    from the beginning or end of a line.

git-svn-id: http://svn.apache.org/repos/asf/subversion/trunk/subversion/tests/cmdline@872537 13f79535-47bb-0310-9956-ffa450edef68