fix compaction split bug and add 3.1 test

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

mini-lsm-starter/src/key.rs

@@ -22,6 +22,10 @@ impl<T: AsRef<[u8]>> Key<T> {
     pub fn is_empty(&self) -> bool {
         self.0.as_ref().is_empty()
     }
+
+    pub fn for_testing_ts(self) -> u64 {
+        0
+    }
 }
 
 impl Key<Vec<u8>> {
@@ -118,6 +122,10 @@ impl<'a> Key<&'a [u8]> {
     pub fn for_testing_from_slice_no_ts(slice: &'a [u8]) -> Self {
         Self(slice)
     }
+
+    pub fn for_testing_from_slice_with_ts(slice: &'a [u8], _ts: u64) -> Self {
+        Self(slice)
+    }
 }
 
 impl<T: AsRef<[u8]> + Debug> Debug for Key<T> {