Distributed: Enabled availability checking in Distributed test.

The distributed_actor_custom_executor_availability.swift test case was testing
behavior that ultimately depends on availability checking but then disabled
availability checking on the command line. This would break with future changes
to availability checking infrastructure.

Author: tshortli

test/Distributed/Runtime/distributed_actor_custom_executor_availability.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend-emit-module -emit-module-path %t/FakeDistributedActorSystems.swiftmodule -module-name FakeDistributedActorSystems -disable-availability-checking %S/../Inputs/FakeDistributedActorSystems.swift
-// RUN: %target-build-swift -Xfrontend -disable-availability-checking -parse-as-library -I %t %s %S/../Inputs/FakeDistributedActorSystems.swift -o %t/a.out
+// RUN: %target-swift-frontend-emit-module -emit-module-path %t/FakeDistributedActorSystems.swiftmodule -module-name FakeDistributedActorSystems %S/../Inputs/FakeDistributedActorSystems.swift
+// RUN: %target-build-swift -parse-as-library -I %t %s %S/../Inputs/FakeDistributedActorSystems.swift -o %t/a.out
 // RUN: %target-codesign %t/a.out
 // RUN:  %target-run %t/a.out