test runner: switch to pure-Python timeout mechanism

The timeout mechanism has been implemented in python,
so there is no more dependence on external utilities
for timing out an inferior dotest.py process.

Platform-specific bits are implemented for POSIX-like
and Windows platforms.  There is a small class named
ProcessHelper in process_control.py that needs to be
overridden for platforms that fall outside of the
POSIX-like and Windows platforms.

See http://reviews.llvm.org/D13124 for more details.

llvm-svn: 248834