Update Invoice tests to use the new blocks interface, and fix reverse blocks iteration indexing

Changelog-None
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

nostrdb/NdbBlock.swift

@@ -139,6 +139,15 @@ struct NdbBlockGroup: ~Copyable {
             rawTextContent: content
         )
     }
+    
+    /// Parses the note contents on-demand from a specific text.
+    static func parse(content: String) throws(NdbBlocksError) -> Self {
+        guard let metadata = BlocksMetadata.parseContent(content: content) else { throw NdbBlocksError.parseError }
+        return self.init(
+            metadata: .pure(metadata),
+            rawTextContent: content
+        )
+    }
 }
 
 enum MaybeTxn<T: ~Copyable>: ~Copyable {

nostrdb/NonCopyableLinkedList.swift

@@ -38,7 +38,7 @@ struct NonCopyableLinkedList<T: ~Copyable>: ~Copyable {
     
     /// Iterates over each item of the list in reverse, with enumeration support.
     func forEachItemReversed<Y, E: Error>(_ borrowingFunction: ((_ index: Int, _ item: borrowing T) throws(E) -> LoopCommand<Y>)) throws(E) -> Y?  {
-        var indexCounter = count
+        var indexCounter = count - 1
         var cursor: Node? = self.tail
         
         outerLoop: while let nextItem = cursor {