Run F* lax-checking on tests

[maximebuyse]

The tests check that the code generated by hax is stable but there is no guarantee that this code is valid. A good start to make things better would be to run F* lax-checking on the the output. The tests probably already cover some existing bugs that this would allow to identify (for example #1208 fixes a bug that was covered by the tests and this would have been revealed by lax-checking).