PSI Pattern Matching: Add equivalence tests for JetPsiUnifier

idea/testData/unifier/equivalence/declarations/localCallables/localExtensionFunctions.kt

@@ -0,0 +1,24 @@
+class A(val n: Int)
+
+fun test() {
+    <selection>fun foo<T: A>(t: T): Int {
+        fun A.a(n: Int): Int = this.n + n
+        fun A.b(n: Int): Int = this.n - n
+
+        return t.n + A(1).a(2) - A(2).b(1)
+    }</selection>
+
+    fun foo<U: A>(u: U): Int {
+        fun A.x(m: Int): Int = n + m
+        fun A.y(n: Int): Int = this.n - n
+
+        return u.n + A(1).x(2) - A(2).y(1)
+    }
+
+    fun foo<V: A>(v: V): Int {
+        fun A.a(n: Int): Int = this.n + n
+        fun A.b(n: Int): Int = this.n + n
+
+        return v.n + A(1).a(2) - A(2).b(1)
+    }
+}