Fix runtests.py to work with LambdaS5.

All tests with LambdaS5 were aborting.
This patch fixes this issue.

I also added an optimization, used in LambdaS5 tests, which consists in building a snapshot of the LambdaS5 interpreter after it has loaded the environment

Note: this patch depends on https://github.com/brownplt/LambdaS5/pull/41