Fixes #65: VM unit tests about GC broken.

From VM instantiation to first GC ever, the
VM is in a degenerate case that cannot be
reached again after subsequent GC runs.

The two first tests (which check the exact
amount of bytes allocated) must therefore
run a first GC before they measure the
original_size (the amount of bytes allocated
in an "empty" VM).
Merge pull request #66 from sjrd/issue/fix-65

Fixes #65: VM unit tests about GC broken.