Do not include Cobertura and instrumented classes into the junit classpath, unless Cobertura is enabled.

This allows to turn Cobertura off and on at any time.
This avoids "mkdir ${cobertura.home}" that breaks builds at Apache Gump.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1545075 13f79535-47bb-0310-9956-ffa450edef68