[K/N] Fix test t51.kt for special backend checks

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/lower/SpecialBackendChecksTraversal.kt

@@ -335,6 +335,7 @@ private class BackendChecker(
     }
 
     override fun visitField(declaration: IrField) {
+        declaration.acceptChildrenVoid(this)
         if (declaration.isFakeOverride) return // Can't happen now, just trying to be future-proof here.
 
         val parent = declaration.parent

kotlin-native/backend.native/tests/compilerChecks/README.md

@@ -5,7 +5,7 @@ To run tests manually on MacOS,
 - run `runtests.sh` as described below. 
 
 K1: `kotlin-native/backend.native/tests/compilerChecks/runtests.sh -language-version 1.9`
-K2: `kotlin-native/backend.native/tests/compilerChecks/runtests.sh -language-version 2.0`
+K2: `kotlin-native/backend.native/tests/compilerChecks/runtests.sh`
 
 Reference output:
 ```text
@@ -108,8 +108,8 @@ kotlin-native/backend.native/tests/compilerChecks/t5.kt:4:83: error: passing Str
 kotlin-native/backend.native/tests/compilerChecks/t50.kt
 kotlin-native/backend.native/tests/compilerChecks/t50.kt:11:16: error: kotlin.native.concurrent.Worker.execute must take an unbound, non-capturing function or lambda, but captures at:
     kotlin-native/backend.native/tests/compilerChecks/t50.kt:11:54
-kotlin-native/backend.native/tests/compilerChecks/t51.kt
-  MISSING EXPECTED ERROR FOR kotlin-native/backend.native/tests/compilerChecks/t51.kt
+kotlin-native/backend.native/tests/compilerChecks/t51.kt:9:28: error: kotlin.native.concurrent.Worker.execute must take an unbound, non-capturing function or lambda, but captures at:
+    kotlin-native/backend.native/tests/compilerChecks/t51.kt:9:66
 kotlin-native/backend.native/tests/compilerChecks/t52.kt
 kotlin-native/backend.native/tests/compilerChecks/t52.kt:5:5: error: kotlin.native.ref.createCleaner must take an unbound, non-capturing function or lambda, but captures at:
      kotlin-native/backend.native/tests/compilerChecks/t52.kt:5:33: x

kotlin-native/backend.native/tests/compilerChecks/t52.kt

@@ -1,6 +1,6 @@
 import kotlin.native.ref.*
 
-@OptIn(kotlin.ExperimentalStdlibApi::class)
+@OptIn(kotlin.experimental.ExperimentalNativeApi::class)
 fun foo(x: Int) {
     createCleaner(42) { println(x) }
 }

kotlin-native/backend.native/tests/compilerChecks/t53.kt

@@ -4,7 +4,7 @@ class C(val x: Int) {
     fun bar(y: Int) = println(x + y)
 }
 
-@OptIn(kotlin.ExperimentalStdlibApi::class)
+@OptIn(kotlin.experimental.ExperimentalNativeApi::class)
 fun foo(x: Int) {
     createCleaner(42, C(x)::bar)
 }