[lldb] Allow partial completions to fix directory completion.

On the command line we usually insert a space after a completion to indicate that
the completion was successful. After the completion API refactoring, this also
happens with directories which essentially breaks file path completion (as
adding a space terminates the path and starts a new arg). This patch restores the old
behavior by again allowing partial completions. Also extends the iohandler
and SB API tests as the implementation for this is different in Editline
and SB API.

llvm-svn: 370043