tests: Fix thread handling in P2P GO Negotiation test cases

tests: Fix thread handling in P2P GO Negotiation test cases

Some of the error paths in go_neg_pbc() and go_neg_pin() did not wait
for the helper thread to complete processing. This could result in
unexpected behavior when the test case could have exited while the
thread was still performing tasks for the GO Negotiation. This could
result in getting stuck in one of the following test cases with
"go_neg_init_pbc thread caught an exception from p2p_go_neg_init: Group
formation timed out" showing up in the log.

This was hit, e.g., with the following test sequence:
no_go_freq p2p_channel_drv_pref_autogo

Signed-off-by: Jouni Malinen <j@w1.fi>