Add support for "runas" in tests

The test file can contains a #runas directive. It may contains
multiple statements but the last one is the value to test:

 #runas a=1; b=3; myfun(a,b)
=> the return value of myfun is checked

 #runas a=myfun(); b=anotherfun(); a+b
=> the value of `a+b` is checked
Fix test/cases directory. Some are skipped because buggy...