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

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