patch 8.1.1771: options test fails on MS-Windows

Problem:    Options test fails on MS-Windows.
Solution:   Add correct and incorrect values for 'completeslash'.

src/testdir/gen_opt_test.vim

@@ -78,6 +78,7 @@ let test_values = {
       \ 'complete': [['', 'w,b'], ['xxx']],
       \ 'concealcursor': [['', 'n', 'nvic'], ['xxx']],
       \ 'completeopt': [['', 'menu', 'menu,longest'], ['xxx', 'menu,,,longest,']],
+      \ 'completeslash': [['', 'slash', 'backslash'], ['xxx']],
       \ 'cryptmethod': [['', 'zip'], ['xxx']],
       \ 'cscopequickfix': [['', 's-', 's-,c+,e0'], ['xxx', 's,g,d']],
       \ 'debug': [['', 'msg', 'msg', 'beep'], ['xxx']],

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1771,
 /**/
     1770,
 /**/