List apptests as dictionaries, pass optional arguments.

List application tests like other tests as python dicts.
Support an "args" member for shell command line arguments.

BUG=392646
TEST=$ mojo/tools/apptest_runner.py 'mojo/tools/data/apptests' out/Debug
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/810443004