[test] Array: Add availability guard to crash test

Array.init(unsafeUninitializedCapacity:…) back-deploys to the 5.0 stdlib, but the implementation there doesn’t catch buffer reassignment.

Author: lorentey

test/stdlib/Inputs/CommonArrayTests.gyb

@@ -572,6 +572,11 @@ ${Suite}.test("${ArrayType}/init(unsafeUninitializedCapacity:...:)/validCount")
 }
 
 ${Suite}.test("${ArrayType}/init(unsafeUninitializedCapacity:...:)/reassignBuffer") {
+  guard #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) else {
+    // When back-deployed to 5.0, this coding error does not get detected.
+    return
+  }
+
   expectCrashLater()
   let otherBuffer = UnsafeMutableBufferPointer<Int>.allocate(capacity: 1)
   let array = ${ArrayType}<Int>(unsafeUninitializedCapacity: 10) { buffer, _ in