Call v8::V8::Dispose() as part of termination.

This step has always been missing from the pdfium test infrastructure
but was only recently flagged by an update to V8. This will be needed
to roll past V8 version 518d217a0e.

Bug: chromium:1290159
Change-Id: I3a93c998614bd8c8381a101ac53ae77eff7f0867
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/89770
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/testing/v8_initializer.cpp b/testing/v8_initializer.cpp
index e2fdfc5..9b295d0 100644
--- a/testing/v8_initializer.cpp
+++ b/testing/v8_initializer.cpp
@@ -112,5 +112,6 @@
 #ifdef PDF_ENABLE_XFA
   cppgc::ShutdownProcess();
 #endif
+  v8::V8::Dispose();
   v8::V8::DisposePlatform();
 }