auth: should downscope with credential access boundary and call storage apis failed

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---

commit: 7c6e9d9
buildURL: Build Status, Sponge
status: failed

Test output

Expected values to be strictly equal:
false !== true
  + expected - actual

  -false
  +true

AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:
false !== true
at Context.<anonymous> (system-test/downscoping.test.js:65:10)
at processImmediate (internal/timers.js:461:21)</pre></details>

[danielbankhead]

Investigating this, the existing sample was already finicky (not actually instantiating GoogleAuth and implementing it's methods):

nodejs-docs-samples/auth/downscoping.js

Lines 116 to 136 in 7c6e9d9

	const storageOptions = {
	projectId: process.env.GOOGLE_CLOUD_PROJECT,
	authClient: {
	sign: () => {
	Promise.reject('unsupported');
	},
	getCredentials: async () => {
	Promise.reject();
	},
	request: opts => {
	return oauth2Client.request(opts);
	},
	authorizeRequest: async opts => {
	opts = opts || {};
	const url = opts.url || opts.uri;
	const headers = await oauth2Client.getRequestHeaders(url);
	opts.headers = Object.assign(opts.headers || {}, headers);
	return opts;
	},
	},
	};

This would break customers in unexpected ways in production. This PR remedies this issue: #2560

Internal note: b/196442993