java: Enable assertions when doing 'make check'.

It turns out that Java assertions are disabled by default.  You have
to add the 'java -ea' flag to the JVM.  Who knew ..?

Because of this oversight, the tests weren't actually performing the
assertions that we wanted (although in fact none of the assertions
were failing).

This change enables assertions when running the tests.

java/run-java-tests

@@ -1,6 +1,6 @@
 #!/bin/sh -
 # libguestfs Perl bindings
-# Copyright (C) 2009 Red Hat Inc.
+# Copyright (C) 2009-2011 Red Hat Inc.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -20,5 +20,5 @@ set -e
 
 for f in t/*.class; do
   classname=$(basename $f .class)
-  $JAVA -Djava.library.path=.libs $classname
+  $JAVA -Djava.library.path=.libs -ea $classname
 done