test_async does not run in async mode

# Description

It looks like the test in `tests/test_async.py` does not run in async mode. Also, it only checks that the execute response has been accepted, not that the process actually completed.

I'm able to start the test in async mode, but I'm not sure how to get the final response, as the statusLocation does not seem to get updated.

# Environment

- operating system: linux
- Python version: 3.8.12
- PyWPS version: 4.5.2
- source/distribution
- [x] git clone
- [ ] Debian
- [ ] PyPI
- [ ] zip/tar.gz
- [ ] other (please specify):
- web server
- [ ] Apache/mod_wsgi
- [ ] CGI
- [ ] other (please specify):

# Steps to Reproduce

# Additional Information