oeqa/core/case.py: Add OEPTestResultTestCase for ptestresult helpers

oeqa/core/case.py: Add OEPTestResultTestCase for ptestresult helpers

Add the OEPTestResultTestCase class as a mix-in class to provide helper
functions for interacting with ptestresults within the extraresults
object generated by the test case.

This class also provides default compression of log text and log files.

Also add support to resulttool for decoding/decompressing log files
embedded in the test results.

(From OE-Core rev: 06cba9883a5964320969301fd05eeb6bec3e786d)

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>