feat(tests): day4 tests

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

xtask/src/main.rs

@@ -10,6 +10,7 @@ enum CopyTestAction {
     Day1,
     Day2,
     Day3,
+    Day4,
 }
 
 #[derive(clap::Subcommand, Debug)]
@@ -168,6 +169,14 @@ fn copy_test_case(test: CopyTestAction) -> Result<()> {
             )
             .run()?;
         }
+        CopyTestAction::Day4 => {
+            cmd!(
+                "cp",
+                "mini-lsm/src/tests/day4_tests.rs",
+                "mini-lsm-starter/src/tests/day4_tests.rs"
+            )
+            .run()?;
+        }
     }
     Ok(())
 }