pegasus-kickstart: Fix segfault in tracing

If the job forks lots of children then the size of the buffer for the
final invocation record fills up with <proc> tags and causes a segfault.
This fix is just a temporary measure until we implement better output
handling in kickstart.
pegasus-kickstart: Add test case for lots of <proc>

This test case checks to make sure kickstart can handle a job
that invokes lots of processes.