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/zterm.zig

@@ -5,6 +5,7 @@ const size = @import("size.zig");
 
 // public exports
 pub const input = @import("input.zig");
+pub const testing = @import("testing.zig");
 
 pub const App = @import("app.zig").App;
 // App also exports further types once initialized with the user events at compile time:
@@ -28,6 +29,7 @@ pub const Style = @import("style.zig");
 
 test {
     _ = @import("terminal.zig");
+    _ = @import("container.zig");
     _ = @import("queue.zig");
 
     _ = color;