Revive old tests

I brought back the old tests by writing a tiny compat module between fort style tests and ounit. Diff looks scary but it's all mechanical work. A few more points:

* Fixed illegal backslash escapes.
* remove unused env.ml
* All of the old tests pass (yay!) except for the str tests. I've commented the test section in oasis that runs them for now. perhaps @vouillon you can take a look as to why they're failing? I'm not sure who wrote these tests...