[lldb-dap] Addressing orphaned processes in tests. (#166205)
In lldb-dap tests, we sometimes spawn subprocesses directly but do not
always correctly clean them up.
This can cause some tests, like the `TestDAP_disconnect.test_attach` to
hang and not properly respect timeouts.
To fix this, I am passing the `lldbtest.Base.spawnSubprocess` helper to
the adapter client so it can be used spawn subprocesses in a way that we
can ensure they're cleaned up.