Set a large thread stack when running the regex tests in a thread.

For testing ithreads cloning, all the regex tests are run twice. Once
"normally", and once in a child ithread, to verify that all regex
constructions can be cloned.

The recently added tests for backreferences starting with 8 or 9 causes a
lot of C recursion in the child thread, enough to bust the default thread
stack size on (at least) HP-UX. So set a large explicit thread stack size.
It doesn't matter that it's large, as we are only running one child thread.