feat(code): part 3 iterators

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm/src/iterators/tests.rs

@@ -0,0 +1,40 @@
+use anyhow::Result;
+use bytes::Bytes;
+
+use super::impls::StorageIterator;
+
+pub mod merge_iterator_test;
+pub mod two_merge_iterator_test;
+
+#[derive(Clone)]
+pub struct MockIterator {
+    pub data: Vec<(Bytes, Bytes)>,
+    pub index: usize,
+}
+
+impl MockIterator {
+    pub fn new(data: Vec<(Bytes, Bytes)>) -> Self {
+        Self { data, index: 0 }
+    }
+}
+
+impl StorageIterator for MockIterator {
+    fn next(&mut self) -> Result<()> {
+        if self.index < self.data.len() {
+            self.index += 1;
+        }
+        Ok(())
+    }
+
+    fn key(&self) -> &[u8] {
+        self.data[self.index].0.as_ref()
+    }
+
+    fn value(&self) -> &[u8] {
+        self.data[self.index].1.as_ref()
+    }
+
+    fn is_valid(&self) -> bool {
+        self.index < self.data.len()
+    }
+}