[6.2] Skip a part of isolated_conformance.swift on WASI

Custom global executor is not landing in 6.2, so we skip the test
by just gating it behind the platform check.

Author: kateinoigakukun

test/Concurrency/Runtime/isolated_conformance.swift

@@ -200,11 +200,9 @@ await Task.detached { @SomeGlobalActor in
 
 // CHECK: Testing a separate task off the main actor
 print("Testing a separate task off the main actor")
+#if !os(WASI)
 await Task.detached {
   if #available(SwiftStdlib 6.2, *) {
-    // Skip tests on platforms that use the same executor for the main
-    // actor and the global concurrent executor.
-    guard Task.defaultExecutor !== MainActor.executor else { return }
 
     precondition(!tryCastToP(mc))
     precondition(!tryCastToP(wrappedMC))
@@ -218,6 +216,7 @@ await Task.detached {
     print("Cast succeeds, but shouldn't")
   }
 }.value
+#endif
 
 
 // Ensure that we access mc later