DERBY-6225 SecureServerTest assertion error on contents of stdout from subprocess

Patch changing the order of

a) the waiting for the process to finish
   starting and
b) read its standard output.

This is done to avoid a race condition in which we read the output
before it has been printed by the subprocess.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1485936 13f79535-47bb-0310-9956-ffa450edef68