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/CompleteImportedFunction/CompleteImportedFunction.dependency.kt

@@ -0,0 +1,3 @@
+package second
+
+fun testFun() : Int = 12

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

@@ -0,0 +1,9 @@
+package first
+
+import second.testFun
+
+fun test() {
+  te<caret>
+}
+
+// EXIST: testFun