[TESTBUG] tools/launcher/Settings.java -Xss size is too small

The test tries to run with -Xss256k, which is fine, but then with -Xss256000, which is too small for the aarch64 thunder devices. They require a minimum stack of 256k, else you get the following error message:

  The Java thread stack size specified is too small. Specify at least 256k
  Error: Could not create the Java Virtual Machine.
  Error: A fatal exception has occurred. Program will exit.

Currently this bug is masked by JDK-8175342, which causes an assert. When I started testing with the fix for JDK-8175342, I started seeing this issue.

Note JDK-7148499 reported the same issue and was fixed in 8, but that bumped the size from 128k to 256k and 128000 to 256000. I think the proper fix is to simply change 256000 to 262144 (256*1024).