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.

examples/demo.zig

@@ -74,10 +74,7 @@ pub fn main() !void {
 
     var container = try App.Container.init(allocator, .{
         .border = .{
-            .separator = .{
-                .enabled = true,
-                .line = .double,
-            },
+            .separator = .{ .enabled = true },
         },
         .layout = .{
             .gap = 2,