Add tests for getCursorGlyph() helper function

src/renderer/cell.zig

@@ -361,14 +361,17 @@ test Contents {
     };
     c.setCursor(cursor_cell, .block);
     try testing.expectEqual(cursor_cell, c.fg_rows.lists[0].items[0]);
+    try testing.expectEqual(cursor_cell, c.getCursorGlyph().?);
 
     // And remove it.
     c.setCursor(null, null);
     try testing.expectEqual(0, c.fg_rows.lists[0].items.len);
+    try testing.expect(c.getCursorGlyph() == null);
 
     // Add a hollow cursor.
     c.setCursor(cursor_cell, .block_hollow);
     try testing.expectEqual(cursor_cell, c.fg_rows.lists[rows + 1].items[0]);
+    try testing.expectEqual(cursor_cell, c.getCursorGlyph().?);
 }
 
 test "Contents clear retains other content" {