[Native/Tests] Add explicit import for SharedImmutable

This fixes an overload resolution ambiguity in K2.

#KT-56272

kotlin-native/backend.native/tests/build.gradle

@@ -884,8 +884,7 @@ standaloneTest("cleaner_in_tls_worker") {
 }
 
 standaloneTest("worker_bound_reference0") {
-    enabled = (project.testTarget != 'wasm32') && // Workers need pthreads.
-            !isK2(project) // KT-56272
+    enabled = (project.testTarget != 'wasm32') // Workers need pthreads.
     source = "runtime/concurrent/worker_bound_reference0.kt"
     flags = ['-tr']
 

kotlin-native/backend.native/tests/runtime/concurrent/worker_bound_reference0.kt

@@ -9,6 +9,7 @@ package runtime.concurrent.worker_bound_reference0
 import kotlin.test.*
 
 import kotlin.native.concurrent.*
+import kotlin.native.concurrent.SharedImmutable
 import kotlin.native.*
 import kotlin.native.ref.WeakReference
 import kotlin.native.runtime.GC