RFR(XS): 8251121: six SA tests leave core files behind on macOS

[Chris Plummer]

Hello,

Please review the following:

https://bugs.openjdk.java.net/browse/JDK-8251121
http://cr.openjdk.java.net/~cjplummer/8251121/webrev.00/index.html

On OSX (and possibly some linux systems), core files are not produced in 
the cwd, but instead end up in some well known location. For OSX it is 
the /cores directory. The core files tend to accumulate there. This 
fixes the core file accumulation problem by moving the core file into 
the cwd, allowing jtreg to manage it. By default jtreg will delete the 
core if the test passes, and retain if if the test fails or RETAIN=all 
is specified.

I got rid of the code in ClhsdbCDSCore.java that explicitly deletes the 
core file because we don't want it deleted if RETAIN=all is used.

thanks,

Chris