client/test: use unique port number

Modify Explorer tests to use a unique port number for the started
application. This should prevent conflicts with other processes that
may be listening on the port 3000 or 3030.

Requires https://github.com/strongloop/loopback-workspace/pull/150

This PR should fix the build failure on Jenkins.

/to @ritch please review
/cc @seanbrookes @rmg @anthonyettinger