Fix failing tests on Windows

To simulate failures opening a cache file, fixtures are written without the read permission bits.
Since Unix permission bits have no meaning on Windows, a slightly more complicated solution is required to achieve the same permissions.
Thankfully, there's a library to abstract that already.

vendor/github.com/hectane/go-acl/posix.go

@@ -0,0 +1,8 @@
+//+build !windows
+
+package acl
+
+import "os"
+
+// Chmod is os.Chmod.
+var Chmod = os.Chmod