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 piper redirect from stderr
to stdout when invoking elixir(c) on windows.
Grouped some windows functionality in test_helper for elixir
Cleaned up the elixir test_helper.exs a bit
Merge pull request #1322 from ToJans/master

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