CP-2737 Return a Future from expect() calls to allow consumers to wait for an expectation to be satisfied

[evanweible-wf]

// This would setup the expectation and carry on with the rest of the test immediately.
MockTransports.http.expect('GET', Uri.parse('/example'));

// This would setup the expectation and wait for it to be satisfied before continuing
// with the rest of the test.
await MockTransports.http.expect('GET', Uri.parse('/example'));

[rmconsole3-wf]

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