Fix missing call to destroy library in embedder test.

This was somehow lost along the way to creating embedder tests.  Presumably
this silently leaks in origin/master, but triggers an assert under XFA. Fixing
this on master is a pre-requisite for merging all the embedder test code onto
XFA.

TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/856773003