feat: introduce testing namespace

This will be used for unit tests in Capy. As of now, it's loosely
inspired by dvui's testing capabilities

Author: zenith391

src/testing.zig

@@ -0,0 +1,18 @@
+//! Testing module for Capy applications
+const capy = @import("capy.zig");
+
+pub const VirtualWindow = struct {
+    window: capy.Window,
+
+    pub fn init() !VirtualWindow {
+        const window = try capy.Window.init();
+        return VirtualWindow{ .window = window };
+    }
+
+    pub fn deinit(self: *VirtualWindow) void {
+        self.window.deinit();
+    }
+
+    // TODO: methods: expectFocused, expectNotFocused, pressKey, click, expectVisible,
+    // expectNotVisible, hash (to check if two states are identical or not) ...
+};