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/multifile/ExtensionFunction/ExtensionFunction.kt

@@ -0,0 +1,12 @@
+package first
+
+class FirstClass() {
+}
+
+fun firstFun() {
+    val a = FirstClass()
+    a.<caret>
+}
+
+// INVOCATION_COUNT: 2
+// EXIST: secondExtension