The memory leak is fixed by calling the GC at the end of the tests. The GC collects the WasmModuleWrapper objects, which deallocates WasmModule c++ object. For the mjsunit tests the GC is already called because of the --invoke_weak_callbacks flag.

BUG=chromium:662388

Review-Url: https://codereview.chromium.org/2476643003
Cr-Commit-Position: refs/heads/master@{#40822}