Various cleanups in our testing code
This MR includes some improvements that were discussed in !38 (merged), and two further changes:
- got rid of FTypeCheckerWrapper (we have a new Utils module now)
- avoid testsuite failures when OCAMLRUNPARAM=b
- a HACKING.md file with some documentation on our testsuites
- the frontend program
client/test/TestMidML.exe
is renamed intoclient/bin/midml.exe
(ormidml
)