Correct tests to use the correct context path for ROOT webapp.
Those were printing the warning
"StandardContext.setPath A context path must either be an empty string or start with a '/'"...

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1640361 13f79535-47bb-0310-9956-ffa450edef68
A better message text. Followup to r1640363.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1640365 13f79535-47bb-0310-9956-ffa450edef68