[FIR LT] Unmute some stepping and local variable tests

Corresponding tests were fixed together with #KT-56982, #KT-56913,
#KT-56926

compiler/testData/debug/stepping/whenNullalbeSubject.kt

@@ -1,5 +1,3 @@
-// IGNORE_BACKEND_K2_LIGHT_TREE: JVM_IR
-//   Reason: KT-56755
 // FILE: test.kt
 
 fun box() {
@@ -18,15 +16,15 @@ fun box() {
 }
 
 // EXPECTATIONS JVM JVM_IR
+// test.kt:4 box
+// test.kt:5 box
 // test.kt:6 box
-// test.kt:7 box
-// test.kt:8 box
-// test.kt:11 box
-// test.kt:15 box
-// test.kt:18 box
+// test.kt:9 box
+// test.kt:13 box
+// test.kt:16 box
 
 // EXPECTATIONS JS_IR
-// test.kt:6 box
-// test.kt:7 box
-// test.kt:7 box
-// test.kt:18 box
+// test.kt:4 box
+// test.kt:5 box
+// test.kt:5 box
+// test.kt:16 box