fix(test_options): adapt global-local option to new Nvim default

The test previously depended on the 'statusline' option being empty by
default.  The default 'statusline' value changed in Nvim commit
28e31f5d3d16fac349d5e2b55837afddb822b0f3 (2025-04-22):

    feat(options): default statusline expression #33036

Adjust the test to first read the default (global) value of
'statusline', then change the local value of the option, then re-read
the global value and verify it hasn't changed.

Author: drmikehenry

test/test_window.py

@@ -61,10 +61,11 @@ def test_vars(vim: Nvim) -> None:
 def test_options(vim: Nvim) -> None:
     vim.current.window.options['colorcolumn'] = '4,3'
     assert vim.current.window.options['colorcolumn'] == '4,3'
+    old_global_statusline = vim.options['statusline']
     # global-local option
     vim.current.window.options['statusline'] = 'window-status'
     assert vim.current.window.options['statusline'] == 'window-status'
-    assert vim.options['statusline'] == ''
+    assert vim.options['statusline'] == old_global_statusline
 
     with pytest.raises(KeyError) as excinfo:
         vim.current.window.options['doesnotexist']