Disable diagnostics ANSI color for non-terminal

If stdout is not a terminal device, don't color the output.

Alternative considered:

* Use [IsTTY](https://github.com/dduan/istty).
* Add a auto color option

Author: dduan

Sources/FileCheck/Diagnostics.swift

@@ -1,7 +1,7 @@
 import Foundation
 
 func diagnose(_ kind : DiagnosticKind, at loc : CheckLocation, with message : String, options: FileCheckOptions) {
-  let disableColors = options.contains(.disableColors)
+  let disableColors = options.contains(.disableColors) || isatty(fileno(stdout)) == 1
   if disableColors {
     print("\(kind): \(message)")
   } else {