[8u40] approval request for 8057742: ant clean test should not fail if one or more external test suites are missing

[A. Sundararajan]

Please approve.

Bug: https://bugs.openjdk.java.net/browse/JDK-8057742
jdk8u40 webrev: http://cr.openjdk.java.net/~sundar/8057742/8u40/webrev.00/
jdk9 review thread: 
http://mail.openjdk.java.net/pipermail/nashorn-dev/2014-September/003403.html

Backported from jdk9-dev/nashorn "as is" with no jdk8u40 specific changes.

Thanks
-Sundar