tests: improved testability

tests/snapshots/ui_snapshot__ui_snapshot_basic.snap

@@ -0,0 +1,23 @@
+---
+source: tests/ui_snapshot.rs
+assertion_line: 50
+expression: out
+---
+Buffer {
+    area: Rect { x: 0, y: 0, width: 40, height: 10 },
+    content: [
+        "┌Lines (space to mark, q to quit)──────┐",
+        "│[ ] first                             │",
+        "│[x] second                            │",
+        "│[ ] third                             │",
+        "│                                      │",
+        "│                                      │",
+        "│                                      │",
+        "│                                      │",
+        "│                                      │",
+        "└──────────────────────────────────────┘",
+    ],
+    styles: [
+        x: 0, y: 0, fg: Reset, bg: Reset, underline: Reset, modifier: NONE,
+    ]
+}