add expect call to ensure tryAllocateExecutable actually succeeds

For some reason, running Avian under the SVN version of Valgrind
caused mmap to fail, which caused tryAllocateExecutable to return a
null pointer, which led to a non-obvious crash later on.  Adding an
expect to check the result immediately will at least make it obvious
what went wrong.
Merge pull request #228 from dicej/expect-executable

add expect call to ensure tryAllocateExecutable actually succeeds