Better instrument 067-preemptive-unpark

We're seeing mysterious failures. It would be nice to understand the
timing of various events. Print them on failure.

Bug: 22242462
Test: .../testrunner.py --host -b -t 067-premptive-unpark
Change-Id: I89e9efa9639e742cb7755f478909787dd7629e23
1 file changed