add test for private-to-this visibility in traits

compiler/testData/diagnostics/tests/variance/privateToThis/Trait.kt

@@ -0,0 +1,7 @@
+internal trait Test</*0*/ in I> {
+    private/*private to this*/ final fun foo(): I {
+        throw Exception()
+    }
+
+    private/*private to this*/ final val i: I get() = foo()
+}

compiler/testData/diagnostics/tests/variance/privateToThis/Trait.txt

@@ -0,0 +1,9 @@
+package 
+
+internal trait Test</*0*/ in I> {
+    private/*private to this*/ final val i: I
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    private/*private to this*/ final fun foo(): I
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}