plugins/elements/gsttee.c: Fix flow aggregation of tee. Error out immediately for...

plugins/elements/gsttee.c: Fix flow aggregation of tee. Error out immediately for all flow returns except OK and NOT_...

Original commit message from CVS:
* plugins/elements/gsttee.c: (gst_tee_handle_buffer):
Fix flow aggregation of tee. Error out immediately for all flow returns
except OK and NOT_LINKED, return NOT_LINKED if all pads are not linked
and return OK if at least one pad is linked.
Before we errored out on "fatal" flow returns (i.e. not for WRONG_STATE)
and otherwise returned the flow return of the last pad, which is wrong.
* tests/check/elements/tee.c: (_fake_chain), (_fake_chain_error),
(GST_START_TEST), (tee_suite):
Add unit tests for the flow aggregation.