Add MultiFileHighlightingTest which checks that other files are not parsed during highlighting

idea/testData/multiFileHighlighting/dependencies/enums.kt

@@ -0,0 +1,18 @@
+package enums
+
+trait Base {
+    fun f() {
+    }
+}
+
+enum class E(val i: Int = 0): Base {
+    E1: E() {
+        override fun f() {
+        }
+    }
+    E2: E(3) {
+        override fun f() {
+        }
+    }
+    E3
+}