[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/test/mocks/client.ts

@@ -69,6 +69,12 @@ export class MockClient extends Client {
     this.output.spinner = () => {};
 
     this.scenario = Router();
+
+    this.output.isTTY = true;
+  }
+
+  get outputBuffer() {
+    return this.mockOutput.mock.calls.map(c => c[0]).join('');
   }
 
   async startMockServer() {