Use the `%target-swift-5.X-abi-triple` substitutions to compile the tests for
deployment to the minimum OS versions required for the APIs used in the tests,
instead of disabling availability checking.
This is in order to avoid errors in complete concurrency checking mnode
in distributed funcs, or rather their thunks, as there is isolation
boundary crossing happening when we pass a value to a distributed func.
This is because as we do this, we pass it to a nonisolated thunk:
```
nonisolated func THUNK(param: Thing) {
if remote {
...
} else {
await self.realFunc(param)
}
}
```
So what happens here is that the Thing would become isolated to the
task and we get a bad isolation crossing as we pass it along to the
"real func".
Sending values into the distributed thunk is the right thing to do to
resolve this problem: `nonisolated func THUNK(param: sending Thing) {}`
Resolves rdar://126577527
