vim-patch:9.1.0161: expand() removes slash after env variable that ends with colon (#27791)

Problem:  expand() removes a slash after an environment variable that
          ends with a colon on Windows.
Solution: Check the correct char for a colon (zeertzjq)

closes: vim/vim#14161

Note: Vim still removes the path-separator at the end, if another path separator
follows directly after it, e.g. on:

```
    echo $FOO='/usr/'
    echo expand('$FOO/bar') == '/usr/bar'
```

see:

,----[ misc1.c:1630 ]
|   // if var[] ends in a path separator and tail[] starts
|   // with it, skip a character
|   if (after_pathsep(dst, dst + c)
| #if defined(BACKSLASH_IN_FILENAME) || defined(AMIGA)
| 	  && (dst == save_dst || dst[-1] != ':')
| #endif
| 	  && vim_ispathsep(*tail))
|       ++tail;
`----

https://github.com/vim/vim/commit/13a014452a7a020a119ac555a690c65b41f3126d

Cherry-pick test_expand.vim change from patch 9.0.1257.

1 files changed, 15 insertions, 2 deletions

diff --git a/test/old/testdir/test_expand.vim b/test/old/testdir/test_expand.vim
index cd537f4ea1..24df156386 100644
--- a/test/old/testdir/test_expand.vim
+++ b/test/old/testdir/test_expand.vim
@@ -45,12 +45,25 @@ endfunc
 
 func Test_expand_tilde_filename()
   split ~
-  call assert_equal('~', expand('%')) 
+  call assert_equal('~', expand('%'))
   call assert_notequal(expand('%:p'), expand('~/'))
-  call assert_match('\~', expand('%:p')) 
+  call assert_match('\~', expand('%:p'))
   bwipe!
 endfunc
 
+func Test_expand_env_pathsep()
+  let $FOO = './foo'
+  call assert_equal('./foo/bar', expand('$FOO/bar'))
+  let $FOO = './foo/'
+  call assert_equal('./foo/bar', expand('$FOO/bar'))
+  let $FOO = 'C:'
+  call assert_equal('C:/bar', expand('$FOO/bar'))
+  let $FOO = 'C:/'
+  call assert_equal('C:/bar', expand('$FOO/bar'))
+
+  unlet $FOO
+endfunc
+
 func Test_expandcmd()
   let $FOO = 'Test'
   call assert_equal('e x/Test/y', expandcmd('e x/$FOO/y'))