test_buffer: don't depend on version-dependent default values of 'def…

…ine'

`test_options` has an assertion that tests whether setting a local value
modifies the global value, but tests it against a hardcoded constant.

This constant changed in neovim/neovim@fcfe535

and
82a2e14
fixes it by using the new constant instead, but this causes tests to
fail against even the latest released version of neovim.

To make this version-agnostic, save the old global value, and assert
that the new value is equal after setting the local value.

Signed-off-by: Michel Alexandre Salim <salimma@fedoraproject.org>

test/test_buffer.py

@@ -103,10 +103,11 @@ def test_options(vim: Nvim) -> None:
     vim.current.buffer.options['shiftwidth'] = 4
     assert vim.current.buffer.options['shiftwidth'] == 4
     # global-local option
+    global_define = vim.options['define']
     vim.current.buffer.options['define'] = 'test'
     assert vim.current.buffer.options['define'] == 'test'
     # Doesn't change the global value
-    assert vim.options['define'] == ''
+    assert vim.options['define'] == global_define
 
     with pytest.raises(KeyError) as excinfo:
         vim.current.buffer.options['doesnotexist']