JDK-8238204: run_tests.sh fails on macOS when called from test_jpackage.sh

[Alexey Semenyuk]

Please review fix [2] for jpackage bug [1].

Replace xargs call with --no-run-if-empty parameter with bash 
expressions in run_tests.sh
Call run_tests.sh from test_jpackager.sh in a way to avoid shebang 
interpretation that doesn't work properly in the environment without 
bash in the PATH.

Both scripts are used to run jpackage tests in local builds.

- Alexey

[1] https://bugs.openjdk.java.net/browse/JDK-8238204

[2] http://cr.openjdk.java.net/~asemenyuk/8238204/webrev.00