waitForProcessOutput should also waitFor internally

Related to GROOVY-3796, there is a race condition where someone could use waitForProcessOutput() and then call exitValue and get an 'java.lang.IllegalThreadStateException: process hasn't exited' error, since the actual process itself hasn't terminated. This can be worked around with a process.waitForProcessOutput(); process.waitFor(), but this is confusing since waitForProcessOutput also implies that we wait for everything.

This patch adds a specific waitFor() to waitForProcessOutput()