This reduces the amount of special paths for testing.
Setup the memory used for testing exactly the same way as in real world.
Also, always connect the interpreter to the instance being executed,
and to the existing WasmInstance struct. This keeps information
synchronized between interpreter and test runner.
These changes allow us to execute e.g. GrowMemory from cctests either
in the interpreter or in compiled code.

R=ahaas@chromium.org

Change-Id: Id4726d061f3cdba789275350f500d769d27d2d63
Reviewed-on: https://chromium-review.googlesource.com/488561
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44966}