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/NotImportedJavaClass/javapackage/Bar.java

@@ -0,0 +1,6 @@
+package javapackage;
+
+// For NotImportedJavaClass.kt
+public class Bar {
+    public static int BAZZ = 1;
+}