test615: fix for Cygwin, unignore in CI
Setting a server-side file read-only by `chmod 0444` has does not
prevent overwriting it via SFTP upload (as tested in CI).
Fix it by setting its MS-DOS read-only attribute in addition. It
requires the Cygwin tool `chattr`.
Also unignore in CI.
Fixes:
```
test 0615...[SFTP put remote failure]
curl returned 0, when expecting 9
615: exit FAILED
=== Start of file stderr615
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0
100 30 0 0 100 30 0 93 --:--:-- --:--:-- --:--:-- 95
100 30 0 0 100 30 0 92 --:--:-- --:--:-- --:--:-- 92
=== End of file stderr615
```
Ref: https://github.com/curl/curl/actions/runs/
14037991918/job/
39300723214#step:12:1269
Closes #16818