Test for crashes from unusually-shaped tasks

[djmitche]

TaskChampion allows any string:string map in a task. Taskwarrior should do something sensible with any such input.

Let's add some tests for this, probably fixing some of the worst related bugs first.

[djmitche]

The main place we see issues is in reports or commands like info, so I think the tests will need to run such commands.

I got a start at this in a .cpp test method, but it seems rather difficult to get everything set up to run commands without actually handing over control of the process to Context -- Context::initialize appears to be required, but also makes a lot of global assumptions, trying to load .taskrc and so on.

I then got a start at a Python test, but I would need some way to create arbitrary TaskChampion tasks -- Taskwarrior is pretty effective at not making tasks it cannot understand. I thought of making a small make_tc_task binary that could be run from Python and would basically call create_task and task_data.set based on the command-line arguments. However, there's no way to create a binary during a run of make test, so this requires a bit of a hack to get it built.