Allow dosep.py to print dotest.py output on success.

Previously all test output was reported by each individual
instance of dotest.py.  After a recent patch, dosep gets dotest
outptu via a pipe, and selectively decides which output to
print.

This breaks certain scripts which rely on having full output
of each dotest instance to do various parsing and/or log-scraping.

While we make no promises about the format of dotest output, it's
easy to restore this to the old behavior for now, although it is
behind a flag.  To re-enable full output, run dosep.py with the -s
option.

Differential Revision: http://reviews.llvm.org/D11816
Reviewed By: Chaoren Lin

llvm-svn: 244469