add(testing): new namespace containing testing capabilities for zterm

The namespace shall also be used for testing the rendering of
`Container`s and `Element`s (including the `Scrollable` element).

The testing renderer currently is a striped down version of the double
buffered render without the secondary buffer and the flushing to stdout.
The internal `Cell` slice (the *screen*) is used for equality checks.

The testing namespace shall provide a way to describe the expected
`Cell` slices that should be validated against.

src/container.zig

@@ -431,3 +431,9 @@ pub fn Container(comptime Event: type) type {
         }
     };
 }
+
+test {
+    _ = Border;
+    _ = Layout;
+    _ = Rectangle;
+}