tests: fix pipelines_parse_launch.delayed_link flakiness

Fixes #345 (closed)

There were two causes for the flakiness, one much rarer than the other.

The test sets up a source with a sometimes pad added during the transition of a wrapper bin from READY to PAUSED.

It runs 4 iterations, the last of which makes it so the negotiation fails.

In that case, the intention as correctly presented by the following comment:

/* [..] ie, the pipeline should create ok but fail to change state */

However the implementation of run_delayed_test was neither calling get_state on the pipeline (it called it on the wrapper bin), nor checking that the return of get_state was FAILURE (it actually checked that it was not).

This led to an obvious race condition, and was fixed by calling get_state on the pipeline, then checking that in this specific case (expect_link == FALSE), the state change has actually failed.

The second, rarer race condition is at set_state time. When we don't expect the link to succeed, the return of set_state may either be FAILURE or ASYNC, depending on timing. This was fixed by taking expect_link into account when checking the return value of set_state.