[Test] KT-61360: add tests for the IrFakeOverrideBuilder

Add tests for fake overrides with focus on java interoperability

Co-authored-by: Aleksandra Arsenteva <aleksandra.arsenteva@jetbrains.com>

compiler/testData/ir/irText/fakeOverrides/vararg/substitutionOnVarargOverride.kt

@@ -0,0 +1,19 @@
+// TARGET_BACKEND: JVM
+
+// FILE: Java1.java
+public class Java1<T> {
+    public void foo(T... a) {}
+}
+
+// FILE: 1.kt
+class A : Java1<Int>()
+
+class B: Java1<Any>() {
+    override fun foo(vararg a: Any) { }
+}
+
+fun test(a: A, b: B) {
+    a.foo(1,2)
+    a.foo(null)
+    b.foo("", 1)
+}