Remove generated codegen tests, move all testData to box/

A single test file will be generated out of box/ directory

compiler/testData/codegen/box/defaultArguments/function/extentionFunctionWithOneDefArg.kt

@@ -0,0 +1,9 @@
+fun Int.foo(a: Int = 1, b: String): Int {
+    return a
+}
+
+fun box(): String  {
+    if (1.foo(b = "b") != 1) return "fail"
+    if (1.foo(2, "b") != 2) return "fail"
+    return "OK"
+}