[RFC] CMake: Capture stderr when running tests.

A possible fix for #1301, based on @Islah's last comment there.
It happens [here](https://github.com/neovim/neovim/blob/master/src/nvim/buffer.c#L1799) and is caused be [these unit tests](https://github.com/neovim/neovim/blob/master/test/unit/buffer_spec.lua#L157-L204) with `eq(-1, ...)`.