test: add tests to catch 2 mutants

The mutants were:
```
MISSED   rusqlite_migration_tokio_async/src/lib.rs:2:10: replace + with * in add in 0.2s build + 0.1s test
MISSED   rusqlite_migration/src/lib.rs:535:35: replace - with + in Migrations<'m>::goto_down in 0.9s build + 1.5s test
```

While we are here, shard mutants to avoid waiting for too long on CI.

.github/workflows/ci.yml

@@ -143,6 +143,9 @@ jobs:
   mutation-tests:
     name: Mutation tests
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        shard: [0, 1, 2, 3]
     steps:
       - uses: actions/checkout@v3
       - uses: actions-rs/toolchain@v1
@@ -155,7 +158,7 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: mutants
-          args: --colors=always
+          args: --colors=always --no-shuffle -vV --shard ${{ matrix.shard }}/4
       - name: Archive results
         uses: actions/upload-artifact@v3
         if: failure()

rusqlite_migration/src/tests/synch.rs

@@ -530,3 +530,26 @@ fn test_user_version_error() {
     assert!(e.is_err(), "{:?}", e);
     insta::assert_debug_snapshot!(e)
 }
+
+#[test]
+fn test_missing_down_migration() {
+    let mut conn = Connection::open_in_memory().unwrap();
+    let ms = vec![
+        M::up("CREATE TABLE t1(a)").down("DROP TABLE t1"),
+        M::up("CREATE TABLE t2(a)").down("DROP TABLE t2"),
+        M::up("CREATE TABLE t3(a)"),
+        M::up("CREATE TABLE t4(a)").down("DROP TABLE t4"),
+        M::up("CREATE TABLE t5(a)"),
+    ];
+
+    let m = Migrations::new(ms);
+    m.to_version(&mut conn, 4).unwrap();
+
+    m.to_version(&mut conn, 3).unwrap();
+    assert_eq!(
+        Err(Error::MigrationDefinition(
+            MigrationDefinitionError::DownNotDefined { migration_index: 2 }
+        )),
+        m.to_version(&mut conn, 2)
+    );
+}

rusqlite_migration_tokio_async/src/lib.rs

@@ -8,7 +8,7 @@ mod tests {
 
     #[test]
     fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
+        let result = add(2, 5);
+        assert_eq!(result, 7);
     }
 }