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/chmod.go

@@ -0,0 +1,38 @@
+//+build windows
+
+package acl
+
+import (
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+// Change the permissions of the specified file. Only the nine
+// least-significant bytes are used, allowing access by the file's owner, the
+// file's group, and everyone else to be explicitly controlled.
+func Chmod(name string, fileMode os.FileMode) error {
+	// https://support.microsoft.com/en-us/help/243330/well-known-security-identifiers-in-windows-operating-systems
+	creatorOwnerSID, err := windows.StringToSid("S-1-3-0")
+	if err != nil {
+		return err
+	}
+	creatorGroupSID, err := windows.StringToSid("S-1-3-1")
+	if err != nil {
+		return err
+	}
+	everyoneSID, err := windows.StringToSid("S-1-1-0")
+	if err != nil {
+		return err
+	}
+
+	mode := uint32(fileMode)
+	return Apply(
+		name,
+		true,
+		false,
+		GrantSid(((mode&0700)<<23)|((mode&0200)<<9), creatorOwnerSID),
+		GrantSid(((mode&0070)<<26)|((mode&0020)<<12), creatorGroupSID),
+		GrantSid(((mode&0007)<<29)|((mode&0002)<<15), everyoneSID),
+	)
+}