Don't write to inferior_ptid in tracectf.c

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* tracectf.c (ctf_target_open): Switch to added thread instead of
	writing to inferior_ptid directly.
	(ctf_target::close): Use switch_to_no_thread instead of writing to
	inferior_ptid directly.