[test] Adjust new Substring test

We don’t have an easy way to check for the runtime version of the stdlib, so a two-way check for behavioral changes isn’t feasible. (Checking for the OS version isn’t good enough.)

Only check for the new behavior, and only when we know for sure that it’s available.

Author: lorentey

test/stdlib/subString.swift

@@ -252,23 +252,21 @@ SubstringTests.test("UTF8View") {
     expectEqual("", String(u.dropFirst(100))!)
     expectEqual("", String(u.dropLast(100))!)
 
-    let expectSubstringWCSIA: Bool
-    // This availability guard should refer to a concrete OS version in
-    // future.
-    if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) {
-      expectSubstringWCSIA = true 
-    } else {
-      expectSubstringWCSIA = false
-    }
-
     checkHasContiguousStorage(s.utf8, expected: true) // Strings always do
-    checkHasContiguousStorage(t, expected: expectSubstringWCSIA)
-    checkHasContiguousStorage(u, expected: expectSubstringWCSIA)
     checkHasContiguousStorageSubstring(t)
     checkHasContiguousStorageSubstring(u)
     checkMatchContiguousStorage(Array(s.utf8), s.utf8, expected: true)
-    checkMatchContiguousStorage(Array(t), t, expected: expectSubstringWCSIA)
-    checkMatchContiguousStorage(Array(u), u, expected: expectSubstringWCSIA)
+
+    // The specialization for Substring.withContiguousStorageIfAvailable was
+    // added in https://github.com/apple/swift/pull/29146.
+    guard #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) else {
+      return
+    }
+
+    checkHasContiguousStorage(t, expected: true)
+    checkHasContiguousStorage(u, expected: true)
+    checkMatchContiguousStorage(Array(t), t, expected: true)
+    checkMatchContiguousStorage(Array(u), u, expected: true)
   }
 }