RFR: CODETOOLS-7902402: Timestamped testing progress in harness.trace

[dmitry.bessonov at oracle.com]

Please review this change which adds time-stamping to the progress 
logged in harness.trace file.

Bug: https://bugs.openjdk.java.net/browse/CODETOOLS-7902402
Webrev: http://cr.openjdk.java.net/~dbessono/7902402/

Timestamping could be disabled by setting 'javatest.trace.skiptimestamp' 
system property to 'true'.

Timestamp format is "yyyy-MM-dd'T'HH:mm:ssZ" (should we allow user to 
change it?)

dmitry