Modify dosep.py to add default session dir parameter

This ensures that all spawned dotest instances store their traces
in the same location.

Test Plan:
run dosep.py with and without a -s option for dotest

cd lldb/test
./dosep.py
./dosep.py -o '-s /tmp/traces'

Differential Revision: http://reviews.llvm.org/D9839

llvm-svn: 237923