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/ExtensionOnNullable/ExtensionOnNullable.kt

@@ -0,0 +1,12 @@
+package first
+
+fun firstFun() {
+    val a : String? = ""
+    a.hello<caret>
+}
+
+// EXIST: helloFun
+// EXIST: helloFunPreventAutoInsert
+// EXIST: helloWithParams
+// ABSENT: helloFake
+// NUMBER: 3