Use i64 for amounts in Rust test vectors

zip_0143.py

@@ -156,7 +156,7 @@ def main():
                 'rust_fmt': lambda x: None if x == -1 else Some(x),
                 }),
             ('hash_type', 'u32'),
-            ('amount', 'u64'),
+            ('amount', 'i64'),
             ('consensus_branch_id', 'u32'),
             ('sighash', '[u8; 32]'),
         ),

zip_0243.py

@@ -163,7 +163,7 @@ def main():
                 'rust_fmt': lambda x: None if x == -1 else Some(x),
                 }),
             ('hash_type', 'u32'),
-            ('amount', 'u64'),
+            ('amount', 'i64'),
             ('consensus_branch_id', 'u32'),
             ('sighash', '[u8; 32]'),
         ),