fix: Run expect tests on Windows with conpty pseudo-terminal (#276)

This brings together a bunch of random, partially implemented packages for support of the new(ish) Windows [`conpty`](https://devblogs.microsoft.com/commandline/windows-command-line-introducing-the-windows-pseudo-console-conpty/) API - such that we can leverage the `expect` style of CLI tests, but in a way that works in Linux/OSX `pty`s and Windows `conpty`.

These include:
- Vendoring the `go-expect` library from Netflix w/ some tweaks to work cross-platform
- Vendoring the `pty` cross-platform implementation from [waypoint-plugin-sdk](b55c787a65/internal/pkg/pty)
- Vendoring the `conpty` Windows-specific implementation from [waypoint-plugin-sdk](b55c787a65/internal/pkg/conpty)
- Adjusting the `pty` interface to work with `go-expect` + the cross-plat version

There were several limitations with the current packages:
- `go-expect` requires the same `os.File` (TTY) for input / output, but `conhost` requires separate file handles
- `conpty` does not handle input, only output
- The cross-platform `pty` didn't expose the full set of primitives needed for `console`

Therefore, the following changes were made:
- Handling of `stdin` was added to the `conpty` interface
- We weren't using the full extent of the `go-expect` interface, so some portions were removed (ie, exec'ing a process) to simplify our implementation and make it easier to extend cross-platform
- Instead of `console` exposing just a `Tty`, it exposes an `InTty` and `OutTty`, to help encapsulate the difference on Windows (on Linux, these point to the same pipe)

Future improvements:
- The `isatty` implementation doesn't support accurate detection of `conhost` pty's without an associated process. In lieu of a more robust check, I've added a `--force-tty` flag intended for test case use - that forces the CLI to run in tty mode.
- It seems the windows implementation doesn't support setting a deadline. This is needed for the expect.Timeout API, but isn't used by us yet.

Fixes #241

cli/root.go

@@ -21,6 +21,7 @@ import (
 
 const (
 	varGlobalConfig = "global-config"
+	varForceTty     = "force-tty"
 )
 
 func Root() *cobra.Command {
@@ -65,6 +66,12 @@ func Root() *cobra.Command {
 	cmd.AddCommand(users())
 
 	cmd.PersistentFlags().String(varGlobalConfig, configdir.LocalConfig("coder"), "Path to the global `coder` config directory")
+	cmd.PersistentFlags().Bool(varForceTty, false, "Force the `coder` command to run as if connected to a TTY")
+	err := cmd.PersistentFlags().MarkHidden(varForceTty)
+	if err != nil {
+		// This should never return an error, because we just added the `--force-tty`` flag prior to calling MarkHidden.
+		panic(err)
+	}
 
 	return cmd
 }
@@ -113,7 +120,16 @@ func createConfig(cmd *cobra.Command) config.Root {
 // isTTY returns whether the passed reader is a TTY or not.
 // This accepts a reader to work with Cobra's "InOrStdin"
 // function for simple testing.
-func isTTY(reader io.Reader) bool {
+func isTTY(cmd *cobra.Command) bool {
+	// If the `--force-tty` command is available, and set,
+	// assume we're in a tty. This is primarily for cases on Windows
+	// where we may not be able to reliably detect this automatically (ie, tests)
+	forceTty, err := cmd.Flags().GetBool(varForceTty)
+	if forceTty && err == nil {
+		return true
+	}
+
+	reader := cmd.InOrStdin()
 	file, ok := reader.(*os.File)
 	if !ok {
 		return false