Tests on windows now intercept stderr from elixir(c) invocations

Previously tests on windows did not intercept stderr when they were
invoked through `:os.cmd`, so we now add a pipe redirect from `stderr`
to `stdout` when invoking elixir(c) on windows.
![image](https://f.cloud.github.com/assets/155977/694658/2774182a-dcb6-11e2-97f2-5a6cabd53c92.png)

Fixes some more issues from #1280.