Add doctest module & TestCase, TestSuite & DocTestOutput classes

Refs #9874
Parses documents where all tests passed.

Refs #9874
Add unit test for all passed case and fixed problems unit test found.

Refs #9874
Parser and tests for all pass and all failure cases.

Refs #9874
Finish parsing of mixed pass/fail document.

Refs #9874
Write out xunit XML when Sphinx build has finished.

This only happens if the builder type is set to doctest.
Refs #9874
Add a test for multi-document output.

Refs #9874
Change xunit output file to match those produced by ctest

Refs #9874
Merge branch 'feature/9874_doctest_xunit_output' into develop