[Test] Update tests according to KT-58544

There are some cases for fake-override actualization which become
  prohibited in the new expect/actual model, so few tests start to fail,
  despite the fact that they are checking entirely different things

compiler/testData/codegen/box/multiplatform/k2/basic/nonExternalEquals.kt

@@ -21,7 +21,7 @@ actual open class Derived : Base() {
     override fun toString() = "Derived"
 
     // no operator modifier is legal because Base.plus is not expect
-    override fun plus(b: Base) = Derived()
+    override fun plus(b: Base): Base = Derived()
 }
 
 fun box() = "OK"

compiler/testData/codegen/box/multiplatform/k2/defaultArguments/bothInExpectAndActual2.kt

@@ -19,15 +19,17 @@ actual interface I {
     actual fun test(source: String = "actual")
 }
 
-actual interface J : I {
+actual interface J : I
+
+interface K : J {
     override fun test(source: String) {
         if (source != "actual") throw AssertionError(source)
     }
 }
 
-class K : J
+class L : K
 
 fun box(): String {
-    K().test()
+    L().test()
     return "OK"
 }