Ignore SIGPIPE in the mini-driver used by these tests.

We're seeing some very occasional failures in these tests where the
mini-driver dies with a SIGPIPE.  We don't use SIGPIPE for anything, and
the main lldb driver program already ignores SIGPIPE, so ignoring it in
the mini-driver is a good way to remove these spurious failures.

<rdar://problem/29740488>

llvm-svn: 290216