[test] make back-deployment testing possible

Author: glessard

test/stdlib/UnsafeRawBufferPointer.swift

@@ -527,6 +527,7 @@ UnsafeRawBufferPointerTestSuite.test("load.unaligned")
   if #available(SwiftStdlib 5.7, *) { return false } else { return true }
 }, reason: "Requires stdlib from Swift 5.7"))
 .code {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   var data: [UInt8] = [0, 0, 0, .max, .max, .max, .max, 0]
   let i = data.firstIndex(of: .max)!
   data.withUnsafeBytes {

test/stdlib/UnsafeRawPointer.swift

@@ -100,6 +100,7 @@ UnsafeMutableRawPointerExtraTestSuite.test("load.unaligned")
   return true
 }, reason: "Requires Swift 5.7's stdlib"))
 .code {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   var data: [UInt8] = [0x0, 0x0, 0x0, 0xff, 0xff, 0xff, 0xff, 0x0]
   var result: UInt32
   result = data.withUnsafeBytes {