add testing tools to variable tab

You can specify a condition on the set of question variables which must pass. The generator should re-run until the condition passes (or it's run too many times, and everything fails).

There's a button to test the condition, which generates variables as many times as possible in 3 seconds, and presents some stats on how often it succeeds, how many runs you'd expect to take, etc.
more control over variables test

You can specify how long (in seconds) to run the variables test for.
The stats summary gives a 95% confidence interval for Pr(success).
The stats summary is cleared as soon as any variable or function changes, or the condition changes.
The code checks for errors when compiling or applying the condition.
The "Running for X seconds..." message counts down, so you know it hasn't crashed.
Merge branch 'master' into test-variables
update math.js