[lldb] Fix TestCompletion by using SIGPIPE instead of SIGINT as test signal

The test I added in commit 07800348 was using
SIGINT for testing the tab completion. The idea is to have a signal that only
has one possible completion and I ended up picking SIGIN -> SIGINT for the test.
However on non-Linux systems there is SIGINFO which is a valid completion for
`SIGIN' and so the test fails there.

This replaces SIGIN -> SIGINT with SIGPIP -> SIGPIPE completion which according
to LLDB's signal list in Host.cpp is the only valid completion.