[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/commands/inspect.test.ts

@@ -23,7 +23,7 @@ describe('inspect', () => {
     client.setArgv('inspect', 'bad.com');
     const exitCode = await inspect(client);
     expect(exitCode).toEqual(1);
-    expect(client.mockOutput.mock.calls[0][0]).toEqual(
+    expect(client.outputBuffer).toEqual(
       `Error! Failed to find deployment "bad.com" in ${user.username}\n`
     );
   });