IR text tests: Add test with the implementation of a private interface

compiler/testData/ir/irText/classes/privateInterface.kt

@@ -0,0 +1,20 @@
+// FIR_IDENTICAL
+private interface I {
+    fun fooString(): String
+    fun barString(): String = "bar@I"
+    fun bazString(): String = "baz@I"
+
+    fun fooUnit(): Unit
+    fun barUnit() {}
+    fun bazUnit() {}
+}
+
+open class C1 : I {
+    override fun fooString() = "foo@C1"
+    override fun barString() = "bar@C1"
+
+    override fun fooUnit() {}
+    override fun barUnit() {}
+}
+
+class C2 : C1()