patch 8.1.2234: get_short_pathname() fails depending on encoding

Problem:    get_short_pathname() fails depending on encoding.
Solution:   Use the wide version of the library function. (closes #5129)
diff --git a/src/testdir/test_shortpathname.vim b/src/testdir/test_shortpathname.vim
index b884820..6b3f85f 100644
--- a/src/testdir/test_shortpathname.vim
+++ b/src/testdir/test_shortpathname.vim
@@ -1,6 +1,9 @@
 " Test for shortpathname ':8' extension.
 " Only for use on Win32 systems!
 
+set encoding=utf-8
+scriptencoding utf-8
+
 source check.vim
 CheckMSWindows
 
@@ -67,3 +70,20 @@
 
   exe "cd " . save_dir
 endfunc
+
+func Test_ColonEight_MultiByte()
+  let dir = 'Xtest'
+
+  let file = dir . '/日本語のファイル.txt'
+
+  call mkdir(dir)
+  call writefile([], file)
+
+  let sfile = fnamemodify(file, ':8')
+
+  call assert_notequal(file, sfile)
+  call assert_match('\~', sfile)
+
+  call delete(file)
+  call delete(dir, 'd')
+endfunc