[test] adjust test values

Author: glessard

test/stdlib/Span/BridgedStringUTF8ViewSpanTests.swift

@@ -28,7 +28,9 @@ let strings = [
   "A long ASCII string exceeding 16 code units.",
   "🇯🇵",
   "🏂☃❅❆❄︎⛄️❄️",
-  "",
+// Enable the following once the small native string form is supported
+//  "z",
+//  "",
 ]
 
 strings.forEach { expected in