[lldb] [test] Terminate "process connect" connections via kill

Fix the termination of "process connect" (and "gdb-remote") to kill
the process rather than attempting to disconnect the platform.
The latter only results in an error since we did not use "platform
connect", and apparently process-level connections (at least via
gdb-remote) do not really support disconnecting.

Differential Revision: https://reviews.llvm.org/D110996