Add tests for obsolete issue

#KT-7412 Obsolete

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

@@ -0,0 +1,10 @@
+open class Foo {
+    open fun foo(x: CharSequence = "O"): CharSequence = x
+}
+class Bar(): Foo() {
+    override fun foo(x: CharSequence): String {   // Note the covariant return type
+        return x.toString() + "K"
+    }
+}
+
+fun box() = Bar().foo()

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

@@ -0,0 +1,10 @@
+open class Foo {
+    open fun foo(x: CharSequence = "O"): CharSequence = x
+}
+class Bar<T : String>: Foo() {
+    override fun foo(x: CharSequence): T {   // Note the covariant return type
+        return (x.toString() + "K") as T
+    }
+}
+
+fun box() = Bar<String>().foo()