Stabilize TestIOHandlerCompletion

pexpect gives as raw data going to a terminal. This means that if the
completed line does not fit the emulated line, the returned data will
contain line breaks. On my machine these line breaks happened to be
inside the "iohandler/completion" string that the test was searching
for.

Work around this by telling pexpect to emulate a very wide terminal.

llvm-svn: 370054