Add BigRAMTester to repo

BigRAMTester is a microbenchmark contributed in JDK-8152438 (see attachments there) that over time has been used a lot to show particular performance deficiencies in various collectors to work on them.

Until now it and its minor extensions of it have been handed around within the gc team internally.

It would be much better to have it available in the openjdk repo for reference, and potentially for stress testing.