failing unit-test

# Description

Attempt to install and run unit-test

The software installs without error, following the documentation - trying to run the test, it fails with the following error:

```
root@5f142103bda1:~/pywps# python3 -m unittest tests
E
======================================================================
ERROR: tests (unittest.loader._FailedTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/root/pywps/tests/__init__.py", line 95, in load_tests
test_storage.load_tests(),
AttributeError: module 'tests.test_storage' has no attribute 'load_tests'

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (errors=1)
```

After commenting the failing import and its related call() to `test_storage`, the rror becomes:

```
python3 -m unittest tests
...................................................EEE.....s.s............................................ss...........................................ss.......
======================================================================
ERROR: test_bbox_output (tests.test_describe.OutputDescriptionTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/root/pywps/tests/test_describe.py", line 385, in test_bbox_output
doc = bbox.describe_xml()
AttributeError: 'BoundingBoxOutput' object has no attribute 'describe_xml'

======================================================================
ERROR: test_complex_output (tests.test_describe.OutputDescriptionTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/root/pywps/tests/test_describe.py", line 368, in test_complex_output
doc = complexo.describe_xml()
AttributeError: 'ComplexOutput' object has no attribute 'describe_xml'

======================================================================
ERROR: test_literal_output (tests.test_describe.OutputDescriptionTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/root/pywps/tests/test_describe.py", line 343, in test_literal_output
doc = literal.describe_xml()
AttributeError: 'LiteralOutput' object has no attribute 'describe_xml'

----------------------------------------------------------------------
Ran 160 tests in 6.500s

FAILED (errors=3, skipped=6)
root@5f142103bda1:~/pywps#
```

# Environment

- operating system: `Debian SID`
- Python version: `3.9.10`
- PyWPS version: `git master`
- 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
follow the documentation to run unit-test after successful installation

# Additional Information