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.

build.zig

@@ -175,6 +175,7 @@ pub fn build(b: *std.Build) void {
         .optimize = optimize,
     });
     lib_unit_tests.root_module.addImport("code_point", zg.module("code_point"));
+    lib_unit_tests.root_module.addImport("DisplayWidth", zg.module("DisplayWidth"));
 
     const run_lib_unit_tests = b.addRunArtifact(lib_unit_tests);