testcrypt.py: look past 'opt_' prefix on argument types.

When testcrypt.h lists a function argument as 'opt_val_foo', it means
that the argument is optional in the sense that the C function can
take a null pointer in place of a valid foo, and so the Python wrapper
module should accept None in the corresponding argument slot from the
client code and translate it into the special string "NULL" in the
wire protocol.

This works fine at argument translation time, but the code that reads
testcrypt.h wasn't looking at it, so if you said 'opt_val_foo_suffix'
in place of 'opt_val_foo' (indicating that that argument is optional
_and_ the C function expects it in a translated form), then the
initial pass over testcrypt.h wouldn't strip the _suffix, and would
set up data structures with mismatched type names.

test/testcrypt.py

@@ -190,9 +190,13 @@ def _setup(scope):
     prefix, suffix = "FUNC(", ")"
     valprefix = "val_"
     outprefix = "out_"
+    optprefix = "opt_"
     consprefix = "consumed_"
 
     def trim_argtype(arg):
+        if arg.startswith(optprefix):
+            return optprefix + trim_argtype(arg[len(optprefix):])
+
         if (arg.startswith(valprefix) and
             "_" in arg[len(valprefix):]):
             # Strip suffixes like val_string_asciz