[Isolated conformances] Don't check results on older runtimes

There are some configurations where we aren't able to fully
determine whether isolated conformances are supported in the runtime,
so don't check the negative cases.

Author: DougGregor

test/Concurrency/Runtime/isolated_conformance.swift

@@ -173,13 +173,6 @@ await Task.detached {
     precondition(!tryCastToP(wrappedMany))
   } else {
     print("Cast succeeds, but shouldn't")
-    precondition(tryCastToP(mc))
-    precondition(tryCastToP(wrappedMC))
-
-    if #available(SwiftStdlib 5.9, *) {
-      let wrappedMany = WrapMany(wrapped: (17, mc, "Pack"))
-      precondition(tryCastToP(wrappedMany))
-    }
   }
 }.value