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/DoNotCompleteWithConstraints/DoNotCompleteWithConstraints.dependency.kt

@@ -0,0 +1,16 @@
+package second
+
+open class SomeOther() {}
+
+class SomeTest<T> {
+  fun testingMethod() {
+
+  }
+}
+
+fun <U : SomeOther> SomeTest<U>.testingUnexpectedFunction() {
+}
+
+fun <W : Any> SomeTest<W>.testingExpectedFunction(i : Int) : String {
+  return ""
+}

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

@@ -0,0 +1,15 @@
+package first
+
+trait TestedTrait() {
+}
+
+fun firstFun() {
+    val a = second.SomeTest<TestedTrait>()
+    a.testing<caret>
+}
+
+// EXIST: testingMethod
+// EXIST: testingExpectedFunction
+// ABSENT: testingUnexpectedFunction
+
+// NUMBER: 2