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/JavaInnerClasses/JavaInnerClasses.kt

@@ -2,6 +2,5 @@ fun main(args: Array<String>) {
     println(R.<caret>)
 }
 
-// JAVA_FILE: JavaInnerClasses.java
 // INVOCATION_COUNT: 1
 // EXIST: layout

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

@@ -7,5 +7,4 @@ fun test() {
 
 /* KT-3779, KT-2821 */
 // INVOCATION_COUNT: 2
-// JAVA_FILE: javapackage/Bar.java
 // EXIST: Bar