Fix failing test with bench feature

Using the `bench` feature, `cargo test` was failing since one of the
benchmarks was running into a debug assertion for attempting to access a
line with an index beyond the grid length.

Since this issue was caused by the `len` property not being serialized
and deserialized, the `#[serde(skip)]` attribute has been changed to
`#[serde(default)]`. The ref-test has been edited to include the correct
grid length for proper deserialization.

This fixes #1604.

src/grid/storage.rs

@@ -32,7 +32,7 @@ pub struct Storage<T> {
     /// having to truncate the raw `inner` buffer.
     /// As long as `len` is bigger than `inner`, it is also possible to grow the scrollback buffer
     /// without any additional insertions.
-    #[serde(skip)]
+    #[serde(default)]
     len: usize,
 }