Fix test_csh when running with the original csh

[kulikjak]

This fixes the issue with test_csh when running with the original csh rather than today widely used tcsh.

tcsh on Linux (csh is just a symlink to tcsh) or Solaris:

$ echo "echo 'test'" | tcsh -i
[me@mymachine ~]$ test
[me@mymachine ~]$ exit
$

and csh on Solaris:

$ echo "echo 'test'" | csh -i
mymachine-i386-25280% test
mymachine-i386-25280% $

By adding a forced newline to the prompt, we can be sure that at least one will be there, and thus the prompt won't 'leak' to the next line.

I am wondering whether it makes sense to add a similar test_tcsh test that would explicitly run tcsh. On Linux, it would do the same thing, but on platforms where both versions of the shell are available, it would test both.

Closes #2409

• ran the linter to address style issues (tox -e fix_lint)
• wrote descriptive pull request text

[gaborbernat]

You'll need to fix the CI.

[kulikjak]

It originally failed during Install OS Dependencies stage with:

Failures
 - nushell (exited 1) - nushell not installed. An error occurred during installation:
 The remote server returned an error: (503) Server Unavailable. Service Unavailable
Error: Process completed with exit code 1.

https://github.com/pypa/virtualenv/actions/runs/3053926463/jobs/4925991461

which I don't think can be due to my changes. So I force pushed the changes to restart the tests and try it again, but it seems that it's awaiting your approval now.

[kulikjak]

The one failure again doesn't seem to be related to my change.

[gaborbernat]

Can you add a changelog entry?

[kulikjak]

Sure, done.