Refactor AbstractMultiFileJvmBasicCompletionTest to extend CompletionTestCase

Extract common code from JetFixtureCompletionBaseTestCase to CompletionTestUtil.kt
Reason for this change is that I couldn't get the check that prohibits tree loading to work with fixture test case

idea/testData/completion/basic/multifile/NotImportedObject/NotImportedObject.dependency.kt

@@ -0,0 +1,14 @@
+package test
+
+object NamedObjectTopLevel1
+object NamedObjectTopLevel2
+
+fun some() {
+  object NamedObjectInFun
+}
+
+class Some {
+  class object {
+    NamedObjectInClassObject
+  }
+}

idea/testData/completion/basic/multifile/NotImportedObject/NotImportedObject.kt

@@ -0,0 +1,9 @@
+package first
+
+fun testFun() {
+    NamedObject<caret>
+}
+
+// EXIST: NamedObjectTopLevel1, NamedObjectTopLevel2
+// ABSENT: NamedObjectInClassObject
+// ABSENT: NamedObjectInFun