Most prototype implementations are not fully supported in the
interpreter. This is the case at least for exception handling, simd, and
atomics. Any function can be redirected to the interpreter though,
either by passing --wasm-interpret-all, or by dynamically redirecting to
the interpreter for debugging.
Making the flags experimental keeps the fuzzer from playing around with
these flags.

Drive-by: Refactor tests which explicitly set the prototype flag to use
a new scope for that.

R=ahaas@chromium.org
BUG=chromium:727584

Change-Id: I67da79f579f1ac93c67189afef40c6524bdd4430
Reviewed-on: https://chromium-review.googlesource.com/519402
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45639}