Rewrite test to remove outdated dependencies

[Expyron]

The goal is mainly to remove the dev-dependency on a very old version of mio.

[alexcrichton]

Could you elaborate a bit more on the rationale for this? The purpose of this test is to ensure that using an external event loop is possible, and it looks like your changes effectively make the test busy loop as opposed to use an external event loop?

[Expyron]

Maybe I misunderstood the purpose of this test; my understanding was that it was only intended to test the Multi::assign() API which is not tested anywhere else.

My goal was to remove the dependency to a very old version of mio, and mio-extras, which is not supported anymore.

I intially tried to just update mio, but reproducing the functionality of mio-extras would require making the implementation overly complex for just a single test, so I ended up removing mio altogether.

As a clarification: the new version does not busy loop, but does thread::sleep() respecting the timeout provided by libcurl.