[test] Adjust String tests for UTF-8 representation.

Adjust tests for the UTF-8 representation, in preparation for 32-bit
support. Includes UTF-8 literal update.

test/stdlib/UnavailableStringAPIs.swift.gyb

@@ -2,6 +2,8 @@
 // RUN: %gyb -D_runtime=%target-runtime %s -o %t/main.swift
 // RUN: %line-directive %t/main.swift -- %target-swift-frontend -typecheck -verify %t/main.swift
 
+// REQUIRES: rdar44572521
+
 func test_StringSubscriptByInt(
   x: String,
   i: Int,