Test case generation hangs from time to time

[yallop]

For example, running test_re.ml with --seed 1916785 runs up to the following point:

Regex: ([ba]+)&(b{2,})

and then hangs, apparently stuck in the code that's generating the next sample input.

I don't know whether the next sample will be produced eventually, but the delay is sometimes at least long enough to time-out a Travis run:

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.