Fix Flate Test, also cleanup

-   Remove SkData double-unref to fix assert.
-   Re-enable the test with DEBUG (was disabled due to double-unref assert).
-   SkRandom instead of rand() for thread-safety.
-   General clean up of the test for readability.

Motivation: I am investigating turning on SkFlate::HaveFlate on Windows.

R=mtklein@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/464333002
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