RFR: JDK-8231915: two JDI tests interfere with each other

[Alex Menkov]

Hi all,

please review a small fix for
https://bugs.openjdk.java.net/browse/JDK-8231915
webrev:
http://cr.openjdk.java.net/~amenkov/jdk14/jdwp_test_interference/webrev/
The fix disables "negative" testing for JdwpListenTest.
Negative testing is useful during development, but can cause 
interference with JdwpAttachTest (JdwpAttachTest already has negative 
testing disabled)

--alex