Fix regressions in dotest.py when passing filters or directories.

See https://llvm.org/bugs/show_bug.cgi?id=24708 for details.
Using '-f FILTER' or unnamed arguments (directories) to dotest.py
will now force no-multiprocessing mode.  This stops a bombardment
of test output spam when following the instructions we provide
in a test session trace for rerunning the test.

llvm-svn: 247101