Added tests

idea/testData/completion/basic/common/SubstitutedSignature1.kt

@@ -0,0 +1,6 @@
+fun foo(list: MutableList<String>) {
+    list.<caret>
+}
+
+// EXIST: { itemText: "add", tailText: "(e: String)", typeText: "Boolean" }
+// EXIST: { itemText: "iterator", tailText: "()", typeText: "Iterator<String>" }

idea/testData/completion/basic/common/SubstitutedSignature2.kt

@@ -0,0 +1,11 @@
+open class C<T> {
+    fun foo(t: T): T = t
+}
+
+class B : C<String>() {
+    fun f() {
+        <caret>
+    }
+}
+
+// EXIST: { itemText: "foo", tailText: "(t: String)", typeText: "String" }