[nfc] Eliminate never written to variable warning from a test.

Author: gottesmm

test/Concurrency/sendnonsendable_region_based_sendability.swift

@@ -444,9 +444,9 @@ func test_class_assign_merges(a : A, b : Bool) async {
     let ns0 = NonSendable()
     let ns1 = NonSendable()
 
-    //this should merge ns0 and ns1
-    //TODO: check out the warning here more
-    var box = ClassBox() // expected-warning {{}}
+    // This should merge ns0 and ns1.
+    var box = ClassBox()
+    box = ClassBox()
     box.contents = ns0
     box.contents = ns1