Fix failing tests on Windows.

Test "test_vim.test_command" was failing because of the call to
os.unlink(..), as the file was still open in vim. Adding a conditional
to execute the call only if we are not running under Windows was the
most straight-forward "fix".

Test "test_vim.test_chdir" was failing for two reasons. Firstly because
of missing Windows-specific handling of path names in
find_file_in_path_option in file_search.c in Neovim core - this was
fixed in neovim/neovim#4711. Secondly because although one can indeed
`:cd /` on Windows (or at least, should be able to), a corresponding
call to `:pwd` will echo the root drive of the current working
directory of Neovim.

test/test_vim.py

@@ -21,8 +21,8 @@ def test_command():
     vim.command('normal itesting\npython\napi')
     vim.command('w')
     ok(os.path.isfile(fname))
-    eq(open(fname).read(), 'testing\npython\napi\n')
-    os.unlink(fname)
+    with open(fname) as f:
+      eq(f.read(), 'testing\npython\napi\n')
 
 
 @with_setup
@@ -63,8 +63,10 @@ def test_strwidth():
 @with_setup(setup=cleanup)
 def test_chdir():
     pwd = vim.eval('getcwd()')
+    root = os.path.abspath(os.sep)
+    # We can chdir to '/' on Windows, but then the pwd will be the root drive
     vim.chdir('/')
-    eq(vim.eval('getcwd()'), '/')
+    eq(vim.eval('getcwd()'), root)
     vim.chdir(pwd)
     eq(vim.eval('getcwd()'), pwd)