Fix Ruby tests: Adapt to stricter TXN_ID checks.

* subversion/bindings/swig/ruby/test/test_fs.rb
  (test_transaction): distinuish between malformed and unknown TXN_IDs

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1517532 13f79535-47bb-0310-9956-ffa450edef68