Dispose isolate at end of UNINITIALIZED_TEST

No-try: true
Bug: v8:6886
Change-Id: I994b998e9e5006c6de183328603f3545158bd32c
Reviewed-on: https://chromium-review.googlesource.com/700536Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48292}

test/cctest/heap/test-array-buffer-tracker.cc

@@ -335,6 +335,7 @@ UNINITIALIZED_TEST(ArrayBuffer_SemiSpaceCopyMultipleTasks) {
              Page::FromAddress(buf2->address()));
     heap::GcAndSweep(heap, OLD_SPACE);
   }
+  isolate->Dispose();
 }
 
 TEST(ArrayBuffer_RetainedSizeIncreases) {