Migration to expect/actual: fix multiplatform highlighting tests

idea/testData/multiModuleHighlighting/multiplatform/nestedClassWithoutImpl/common/common.kt

@@ -1,15 +1,15 @@
 package a
 
-header class A {
+expect class A {
     class Nested
 }
 
-header class B {
+expect class B {
     class Nested {
         fun foo(s: String)
     }
 }
 
-header class C {
+expect class C {
     <error>header</error> inner class Inner
 }

idea/testData/multiModuleHighlighting/multiplatform/nestedClassWithoutImpl/jvm/jvm.kt

@@ -1,11 +1,11 @@
 package a
 
-impl class <error>A</error>
+actual class <error>A</error>
 
-impl class B {
-    impl class <error>Nested</error>
+actual class B {
+    actual class <error>Nested</error>
 }
 
-impl class C {
-    impl inner class Inner
+actual class C {
+    actual inner class Inner
 }