The macro tests were all using "REQUIRES: OS=macosx" as a proxy for
"have the Swift Swift parser". There was an existing feature for this,
but it was just checking whether the path was passed through. Fix that
to use the same variable as in CMake.
Also remove all extraneous `-I` and `-L` to the host libs in the target
invocations.
* [Observation] Change visibility of observation runtime functions to be hidden
* [Observation] Update API requirements for Observable AsyncSequence names and alter the behavior of value emissions to be based upon transactionality
* [Observation] Slight naming alteration of isolation -> isolatedTo
Note: This re-enables the previously disabled tests since the implementation should be considerably more robust to hangs.
