perf tests: Fix map_groups refcount test

perf tests: Fix map_groups refcount test

When introducing reference counting for struct thread instances I forgot
to remove the synthetic threads from the machine's rbtree so that it
then the threads would have just one reference and thus the
thread__put() replacing the thread__delete() really turns into a
thread__delete() (thread->refcnt == 1 at thread__put() time) and thus
drop the thread->mg refcount, as expected by the this test.

Fix it by calling machine__remove_thread() (the counterpart of
machine__findnew_thread()) on all the synthetic threads after the
checks that involves the rbtree were done.

Before:

  # perf test -v mg
  30: Test thread mg sharing                                 :
  --- start ---
  test child forked, pid 26995
  FAILED tests/thread-mg-share.c:68 wrong refcnt (4 != 3)
  test child finished with -1
  ---- end ----
  Test thread mg sharing: FAILED!
  #

After:

  # perf test mg
  30: Test thread mg sharing: Ok
  #

Fixes: b91fc39f4ad7 ("perf machine: Protect the machine->threads with a rwlock")
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-uoqq0fjei90ohhhcboz6ay33@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>