Clean up os=wasi in exclusivity_custom_executors.swift

Author: MaxDesiatov

test/Concurrency/Runtime/exclusivity_custom_executors.swift

@@ -12,7 +12,6 @@
 
 // Disabled until test hang can be looked at.
 // UNSUPPORTED: OS=windows-msvc
-// UNSUPPORTED: OS=wasi
 
 // This test makes sure that we properly save/restore access when we
 // synchronously launch a task from a serial executor. The access from the task