Allow effectively overriding isTTY for mocking console output

[heaths]

In order to test CLIs it would be great if

termenv/termenv.go

Line 27 in 6fd0ee9

func (o *Output) isTTY() bool {

could effectively be overridden, such we could force treating Output.tty as a TTY or not. This is useful to provide a buffer such that any writes to an Output could include ANSI sequences, depending on which profile we might force. This way, if a CLI wants to test color output or maybe even varies the output based on whether it's a TTY (say, output TSV if not, or using a text/tablewriter if it is a TTY) it can force it.

E.g.

func Test(t *testing.T) {
 buffer := bytes.Buffer{}
  output = termenv.NewOutput(buffer, WithTTY(true), WithProfile(termenv.TrueColor))
  s := output.String("Hello, world!")
  s.Bold()
  fmt.Println(s);
  want := "\x1b[1mHello, world!\x1b[m"
  if got := buffer.String(); got != want  {
    t.Fatal("want %q; got %q", want, got)
  }
}