feat(test): expose test buffer

Allow a way to expose the buffer of the `TestBackend`,
to easier support different testing methodologies.

src/backend/test.rs

@@ -3,7 +3,7 @@ use crate::{
     buffer::{Buffer, Cell},
     layout::Rect,
 };
-use std::{fmt::Write, io};
+use std::{fmt::{Write, Display}, io};
 use unicode_width::UnicodeWidthStr;
 
 /// A backend used for the integration tests.
@@ -105,6 +105,12 @@ impl TestBackend {
     }
 }
 
+impl Display for TestBackend {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}",buffer_view(&self.buffer))
+    }
+}
+
 impl Backend for TestBackend {
     fn draw<'a, I>(&mut self, content: I) -> Result<(), io::Error>
     where