Auto merge of #3700 - matklad:faster-test, r=alexcrichton
Auto merge of #3700 - matklad:faster-test, r=alexcrichton

Don't use real serde in tests

This test used to take 1.5 minutes on my machine (without significant CPU usage however), now it finishes in a couple of seconds. I don't fully understand what is tested here, but I suppose we don't really need serde.

tsurai/cargo,durka/cargo,eminence/cargo,vojtechkral/cargo,nipunn1313/cargo,tshepang/cargo,rust-lang/cargo,durka/cargo,vojtechkral/cargo,durka/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo,rust-lang/cargo,achanda/cargo,rust-lang/cargo,tsurai/cargo,tshepang/cargo,achanda/cargo,vojtechkral/cargo,eminence/cargo,rust-lang/cargo,achanda/cargo,durka/cargo,rustyhorde/cargo,tsurai/cargo,achanda/cargo,nipunn1313/cargo,tshepang/cargo,vojtechkral/cargo,nipunn1313/cargo,tsurai/cargo,rustyhorde/cargo,tsurai/cargo,rustyhorde/cargo,tshepang/cargo,eminence/cargo