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;
`----
13a014452a
Cherry-pick test_expand.vim change from patch 9.0.1257.
Problem: Test failures because of changed error messages.
Solution: Adjust the exptected error messages.
ec89223478
Co-authored-by: Bram Moolenaar <Bram@vim.org>