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/ExtensionFunctionOnImportedFunction/ExtensionFunctionOnImportedFunction.dependency.kt

@@ -0,0 +1,9 @@
+package second
+
+class Some
+
+// Two function to prevent automatic insert
+fun Some.extensionFunction1(): Int = 12
+fun Some.extensionFunction2(): Int = 12
+
+fun foo(): Some = Some()

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

@@ -0,0 +1,9 @@
+package first
+
+// For KT-3102
+
+import second.foo
+
+fun test() = foo().ext<caret>
+
+// EXIST: extensionFunction1, extensionFunction2