Fix tests: "infix modifier required" and "operator modifier required" errors

compiler/testData/codegen/box/increment/postfixIncrementOnClass.kt

@@ -1,7 +1,7 @@
 interface Base
 class Derived: Base
 class Another: Base
-fun Base.inc(): Derived { return Derived() }
+operator fun Base.inc(): Derived { return Derived() }
 
 public fun box() : String {
     var i : Base

compiler/testData/codegen/box/increment/postfixIncrementOnClassSmartCast.kt

@@ -1,6 +1,6 @@
 open class Base
 class Derived: Base()
-fun Derived.inc(): Derived { return Derived() }
+operator fun Derived.inc(): Derived { return Derived() }
 
 public fun box() : String {
     var i : Base

compiler/testData/codegen/box/increment/postfixNotnullClassIncrement.kt

@@ -1,7 +1,7 @@
 class MyClass
 
 // In principle it is not correct, MyClass? is not a subtype of MyClass
-fun MyClass.inc(): MyClass? = null
+operator fun MyClass.inc(): MyClass? = null
 
 public fun box() : String {
     var i : MyClass? 

compiler/testData/codegen/box/increment/postfixNullableClassIncrement.kt

@@ -1,6 +1,6 @@
 class MyClass
 
-fun MyClass?.inc(): MyClass? = null
+operator fun MyClass?.inc(): MyClass? = null
 
 public fun box() : String {
     var i : MyClass? 

compiler/testData/codegen/box/increment/postfixNullableIncrement.kt

@@ -1,4 +1,4 @@
-fun Int?.inc(): Int? = this
+operator fun Int?.inc(): Int? = this
 
 fun init(): Int? { return 10 }
 

compiler/testData/codegen/box/increment/prefixIncrementOnClass.kt

@@ -1,7 +1,7 @@
 interface Base
 class Derived: Base
 class Another: Base
-fun Base.inc(): Derived { return Derived() }
+operator fun Base.inc(): Derived { return Derived() }
 
 public fun box() : String {
     var i : Base

compiler/testData/codegen/box/increment/prefixIncrementOnClassSmartCast.kt

@@ -1,6 +1,6 @@
 open class Base
 class Derived: Base()
-fun Derived.inc(): Derived { return Derived() }
+operator fun Derived.inc(): Derived { return Derived() }
 
 public fun box() : String {
     var i : Base

compiler/testData/codegen/box/increment/prefixNotnullClassIncrement.kt

@@ -1,7 +1,7 @@
 class MyClass
 
 // In principle it is not correct, MyClass? is not a subtype of MyClass
-fun MyClass.inc(): MyClass? = null
+operator fun MyClass.inc(): MyClass? = null
 
 public fun box() : String {
     var i : MyClass? 

compiler/testData/codegen/box/increment/prefixNullableClassIncrement.kt

@@ -1,6 +1,6 @@
 class MyClass
 
-fun MyClass?.inc(): MyClass? = null
+operator fun MyClass?.inc(): MyClass? = null
 
 public fun box() : String {
     var i : MyClass? 

compiler/testData/codegen/box/increment/prefixNullableIncrement.kt

@@ -1,4 +1,4 @@
-fun Int?.inc(): Int? = this
+operator fun Int?.inc(): Int? = this
 
 fun init(): Int? { return 10 }