RFR 8072906: sun/management/jmxremote/bootstrap/CustomLauncherTest.java failing on embedded platform

[Jaroslav Bachorik]

Please, review the following test change

Issue : https://bugs.openjdk.java.net/browse/JDK-8072906
Webrev: http://cr.openjdk.java.net/~jbachorik/8072906/webrev.00

The test tries to exercise a custom launcher on embedded platforms and 
fails because the corresponding binary is not available. The test should 
not require the presence of the binary and skip the execution gracefully 
if it can't find one - this was broken while addressing JDK-8029809.

The fix brings back the ability just skip the test if the launcher 
binary does not exist.

Thanks,

-JB-