Created module idea-test-framework and moved classes needed for idea tests there

Moved tests for completion and their test data into module idea-completion

idea/idea-completion/testData/basic/common/SubstitutedSignature4.kt

@@ -0,0 +1,14 @@
+trait Trait<T>
+
+fun<T> Trait<T>.extension(t: T): T = t
+
+class Outer : Trait<String> {
+    inner class Inner : Trait<Int> {
+        fun foo() {
+            ext<caret>
+        }
+    }
+}
+
+// EXIST: { itemText: "extension", tailText: "(t: String) for Trait<T> in <root>", typeText: "String" }
+// EXIST: { itemText: "extension", tailText: "(t: Int) for Trait<T> in <root>", typeText: "Int" }