Update test so it doens't leave isolates hanging.
Update test so it doens't leave isolates hanging.

This caused timeouts on Firefox.

Review URL: https://codereview.chromium.org//579913004

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@40539 260f80e4-7a28-3924-810f-c04153c831b5

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