RFR: 8170976: [TESTBUG] LogTestFixture does not restore previous logging state

[Marcus Larsson]

Hi,

Please review the following change to retain log configuration 
throughout gtest runs for UL.

A snapshot will now be taken before each test run (using the 
LogTestFixture), and that configuration is then restored after the test 
completes.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8170976

Webrev:
http://cr.openjdk.java.net/~mlarsson/8170976/webrev.00

Verified that GTest tests still pass.

Thanks,
Marcus