(QENG-845) Beaker junit xml: XML Parsing error

- need to strip out invalid characters from strings before printing to
  xml, these characters are still invalid even if wrapped in a cdata
  tag
@anodelman In a PR I just merged, you altered logic based on `test.stderr` in `test_suite.rb`: https://github.com/puppetlabs/beaker/pull/370/files#diff-6486a758112c118b1899adaa06bc7bbeL243

I'm wondering if similar logic needs to be implemented here? It seems likely that this section of the code should at least be dealing with `test.last_result` rather than `test.stderr`, but I admit I don't really understand the distinction between `test.stderr` and `test.last_result`