[cli] Add outputBuffer getter and mutable isTTY for tests (#6827)

These changes originally from #6652, but pulled out to be merged
separately.

`outputBuffer` is a simpler way of asserting tests against the CLI
output instead of working directly withe Jest mock function.

`output.isTTY` is also now mutable, so that we can write tests for both
cases when the output is different based on TTY-ness (for example,
see the updated `vc whoami` tests in this PR).

packages/cli/src/util/output/create-output.ts

@@ -20,15 +20,13 @@ export class Output {
   private debugEnabled: boolean;
   private spinnerMessage: string;
   private _spinner: StopSpinner | null;
+  isTTY: boolean;
 
   constructor({ debug: debugEnabled = false }: OutputOptions = {}) {
     this.debugEnabled = debugEnabled;
     this.spinnerMessage = '';
     this._spinner = null;
-  }
-
-  get isTTY() {
-    return process.stdout.isTTY;
+    this.isTTY = process.stdout.isTTY || false;
   }
 
   isDebugEnabled = () => {