RFR: 8232841: [TESTBUG] [macos] SigningPackageTest fails when untrusted certificates exist on machine

[alexander.matveev at oracle.com]

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

- I was not able to reproduce this issue. Not sure why this command 
returns non zero code in some cases. It still returns output which we 
need, so fixed by ignoring exit code, since we will scan output for 
presence of certificate anyway.

[1] https://bugs.openjdk.java.net/browse/JDK-8232841

[2] http://cr.openjdk.java.net/~almatvee/8232841/webrev.00/

Thanks,
Alexander