archiver: Move tests back into the same file

Move all Archiver tests back into `archiver_test.go` and add some tiny
helpers to mock what `lstat` returns (for Windows and Unix separately).

internal/archiver/archiver_unix_test.go

@@ -0,0 +1,41 @@
+// +build !windows
+
+package archiver
+
+import (
+	"os"
+	"syscall"
+	"testing"
+)
+
+type wrappedFileInfo struct {
+	os.FileInfo
+	sys  interface{}
+	mode os.FileMode
+}
+
+func (fi wrappedFileInfo) Sys() interface{} {
+	return fi.sys
+}
+
+func (fi wrappedFileInfo) Mode() os.FileMode {
+	return fi.mode
+}
+
+// wrapFileInfo returns a new os.FileInfo with the mode, owner, and group fields changed.
+func wrapFileInfo(t testing.TB, fi os.FileInfo, mode os.FileMode, uid, gid uint) os.FileInfo {
+	// get the underlying stat_t and modify the values
+	stat := fi.Sys().(*syscall.Stat_t)
+	stat.Mode = uint32(mode)
+	stat.Uid = uint32(uid)
+	stat.Gid = uint32(gid)
+
+	// wrap the os.FileInfo so we can return a modified stat_t
+	res := wrappedFileInfo{
+		FileInfo: fi,
+		sys:      stat,
+		mode:     mode,
+	}
+
+	return res
+}