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

idea/testData/multiFileHighlighting/enumReference.kt

@@ -0,0 +1,17 @@
+package some
+
+import enums.E
+import enums.Base
+
+fun f(<warning>e</warning>: Base) {
+
+}
+
+fun test() {
+    f(E.E1)
+    f(E.E2)
+    f(E.E3)
+    f(E.<error>E4</error>)
+    f(E.<error>Invalid</error>)
+    E.E1.f()
+}