This CL also removes the v8::internal::wasm::testing namespace, which
had methods that were defined in wasm-module.h, moving them to be
*ForTesting methods on the corresponding WASM heap objects.

R=clemensh@chromium.org

Bug: v8:6877
Change-Id: I1b346ec64d93ee3a122b8f7e69772cfe98371754
Reviewed-on: https://chromium-review.googlesource.com/695523Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48286}