RFR (T) 8223194: runtime/Shutdown/ShutdownTest.java due to "OutOfMemoryError: Java heap too small"

[coleen.phillimore at oracle.com]

Summary: Use default memory for the test.

This test doesn't start out with enough memory for ZGC.  Rather than 
having to figure out what memory size works, and having that change, 
make the test run with the default amount of heap memory.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8223194.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8223194

Thanks,
Coleen