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

testing/embedder_test.cpp

diff --git a/testing/embedder_test.cpp b/testing/embedder_test.cpp
index e823347..74d4f25 100644
--- a/testing/embedder_test.cpp
+++ b/testing/embedder_test.cpp
@@ -195,6 +195,7 @@
 void EmbedderTest::TearDown() {
   FPDF_CloseDocument(document_);
   FPDFAvail_Destroy(avail_);
+  FPDF_DestroyLibrary();
   if (loader_) {
     delete loader_;
   }