RFR: JDK-8191923 Save run-test summary to file

[Tim Bell]

Magnus:

> We should save the run-test summary to a file.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8191923
> WebRev:
> http://cr.openjdk.java.net/~ihse/JDK-8191923-save-run-test-summary-to-file/webrev.01

Looks good.

/Tim