m/p/erofs: add character device test

Also, drive-by fix a misleading comment.

Change-Id: Ic5dc2d16a0c2f453d55f0f698f06f30fd355098a
Reviewed-on: https://review.monogon.dev/c/monogon/+/649
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/pkg/erofs/inode_types.go b/metropolis/pkg/erofs/inode_types.go
index bac29c5..8147892 100644
--- a/metropolis/pkg/erofs/inode_types.go
+++ b/metropolis/pkg/erofs/inode_types.go
@@ -114,7 +114,7 @@
 	return i
 }
 
-// CharacterDevice represents a Unix block device inode with major and minor
+// BlockDevice represents a Unix block device inode with major and minor
 // numbers.
 type BlockDevice struct {
 	Base