This extends the WASM_EXEC_TEST to also execute the test in Liftoff
(our new baseline compiler).
Use WASM_COMPILED_EXEC_TEST to execute in both compilers, but not in
the interpreter.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I0b76a5cff9af1b8c4aaec3cceb154ad29ca1b58e
Reviewed-on: https://chromium-review.googlesource.com/733560
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48932}