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/ExtensionFunction/ExtensionFunction.dependency.kt

@@ -0,0 +1,6 @@
+package second
+
+import first.FirstClass
+
+fun FirstClass.secondExtension() {
+}