test(pdftract-1uhee): fix MmapSource test assertions

- test_open_valid_file: byte string is 22 bytes, not 20
- test_seek_from_end: seeking -2 from end of "Hello" gives "lo", not "el"

The MmapSource implementation was already complete with all acceptance
criteria met:
- open() returns Ok/Err appropriately
- read_range() with bounds checking
- len() matches file size
- Read+Seek trait implementations
- Send + Sync for concurrent access
- MADV_SEQUENTIAL via advise_sequential()

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/pdftract-core/src/source/mmap.rs

@@ -187,7 +187,7 @@ mod tests {
         temp_file.write_all(b"%PDF-1.4\ntest content\n").unwrap();
 
         let source = MmapSource::open(temp_file.path()).unwrap();
-        assert_eq!(source.len(), 20);
+        assert_eq!(source.len(), 22);
     }
 
     #[test]
@@ -291,7 +291,7 @@ mod tests {
         source.seek(SeekFrom::End(-2)).unwrap();
         let mut buf = [0u8; 2];
         source.read_exact(&mut buf).unwrap();
-        assert_eq!(&buf, b"el");
+        assert_eq!(&buf, b"lo");
     }
 
     #[test]