Fix TestAttachDenied on linux

This test was timing out because the test inferior was forking a child, which was not terminated
correctly. The test contained provisions to terminate this child, but these were no longer
working. The idea was to wake up upon receiving SIGTERM and then kill the child. However, this
was failing because the test first tried to use SIGHUP, which ended up killing the inferior.

Fix: make sure we catch SIGHUP also.
llvm-svn: 248889