Write unit tests with the help of afero

Afero is a package that lets you mock out a filesystem with an in-memory filesystem.
It allows us to easily create the files required for a given test without worrying about
a cleanup step or different tests tripping on eachother when run in parallel.

Later on I'll standardise on using afero over the vanilla os package

vendor/github.com/spf13/afero/const_win_unix.go

@@ -0,0 +1,22 @@
+// Copyright © 2016 Steve Francia <spf@spf13.com>.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//go:build !darwin && !openbsd && !freebsd && !dragonfly && !netbsd && !aix
+// +build !darwin,!openbsd,!freebsd,!dragonfly,!netbsd,!aix
+
+package afero
+
+import (
+	"syscall"
+)
+
+const BADFD = syscall.EBADFD