Fix buildbots after https://reviews.llvm.org/D120755.

This improves this test a lot because before when using the "attachCommands" to run the following commands:

(lldb) target create -d /path/to/a.out
(lldb) process launch

This was racy as it wasn't stopping the program at the entry point, and the process might run to completion before we can even debug it. With the recent changes to the "attachCommands" we were waiting for the process to stop, but the process might be exited already, and that _should_ have caused the attach to fail since there was no process to attach to. By adding "--stop-at-entry" to the process launch, we ensure this should be less racy and give us a valid process to attach to.