The wasm-module-runner is used both in cctests and in fuzzers. As
discussed offline, it is weird to include cctest header files in
fuzzers, so I introduce a new test/common directory which contains the
common files.

R=titzer@chromium.org, jochen@chromium.org

Review-Url: https://codereview.chromium.org/2335193002
Cr-Commit-Position: refs/heads/master@{#39411}