Migrate testdata to new lambda syntax

compiler/testData/codegen/box/regressions/kt3173.kt

@@ -1,4 +1,4 @@
 fun box(): String {
-    val sum = {Int.(other: Int) -> this + other }
+    val sum = fun Int.(other: Int) = this + other
     return if (1 sum 2 == 3) "OK" else "Fail"
 }

compiler/testData/codegen/box/regressions/kt3999.kt

@@ -19,7 +19,7 @@ fun test2() {
 }
 
 fun Int.foo(a: Int) = this * a
-val boo = {Int.(a: Int): Int -> this + a}
+val boo = fun Int.(a: Int): Int = this + a
 
 fun box(): String {
     test1()