patch 8.2.3579: CI sometimes fails for MinGW

Problem:    CI sometimes fails for MinGW.
Solution:   Use backslashes in HandleSwapExists(). (Christian Brabandt,
            closes #9078)

src/testdir/runtest.vim

@@ -93,7 +93,12 @@ set encoding=utf-8
 " REDIR_TEST_TO_NULL has a very permissive SwapExists autocommand which is for
 " the test_name.vim file itself. Replace it here with a more restrictive one,
 " so we still catch mistakes.
+if has("win32")
+  " replace any '/' directory separators by '\\'
+  let s:test_script_fname = substitute(expand('%'), '/', '\\', 'g')
+else
   let s:test_script_fname = expand('%')
+endif
 au! SwapExists * call HandleSwapExists()
 func HandleSwapExists()
   if exists('g:ignoreSwapExists')

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3579,
 /**/
     3578,
 /**/