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()

compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -2778,6 +2778,18 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("covariantOverride.kt")
+            public void testCovariantOverride() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/defaultArguments/function/covariantOverride.kt");
+                doTest(fileName);
+            }
+
+            @TestMetadata("covariantOverrideGeneric.kt")
+            public void testCovariantOverrideGeneric() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/defaultArguments/function/covariantOverrideGeneric.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("extensionFunctionManyArgs.kt")
             public void testExtensionFunctionManyArgs() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/defaultArguments/function/extensionFunctionManyArgs.kt");