fix(timers): Omit fake timers from auto-sandbox

As mentioned in README.md under "Gotchas," relying on useFakeTimers() by default
will also now fake setImmediate. Due to how setImmediate is used in
general, I think this has too much potential for confusing hangs during test runs
due to setImmediate use in either the SUT or down the node_modules
chain.

I'd prefer to avoid that scenario with a new default and accept the cost
of having to manually call this.sandbox.useFakeTimers().
docs(createSandbox): Add fake timers note