Add tool functional test for parallelism with optional inputs.
This functionality is broken without patch from pull request #334. To run this broken test:

% # First two exports are optional but speed up testing and force newest test framework.
% export GALAXY_TEST_DEFAULT_INTERACTOR=api;
% export GALAXY_TEST_DB_TEMPLATE=https://github.com/jmchilton/galaxy-bootstrap/raw/master/src/main/resources/com/github/jmchilton/galaxybootstrap/universe.sqlite
% ./run_functional_tests.sh -framework -id parallelism_optional