Fix TestProcessLaunch breakage on MacOS

This test started failing after r327625. The cause seems difference in the
treatment of relative --stdin paths between MacOS (debugserver?) and linux
(lldb-server?). Linux treats this as relative to the debuggers PWD, while MacOS
as relative to (I think) the future PWD of the launched process.

This fixes the issue by using absolute paths, which should work everywhere, but
we should probably unify this path handling as well. I'll ask around about what
is the expected behavior here.

llvm-svn: 327633