Disable webp tests for now

```
======================================================================
ERROR: test_all_parametred (tests.test_libmat2.TestCleaning.test_all_parametred) (case={'name': 'webp', 'parser': <class 'libmat2.images.WEBPParser'>, 'meta': {'Warning': '[minor] Improper EXIF header'}, 'expected_meta': {}})
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/builds/jvoisin/mat2/libmat2/images.py", line 109, in __init__
    GdkPixbuf.Pixbuf.new_from_file(self.filename)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
gi.repository.GLib.GError: gdk-pixbuf-error-quark: Couldn’t recognize the image file format for file “./tests/data/clean.webp” (3)
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/builds/jvoisin/mat2/tests/test_libmat2.py", line 557, in test_all_parametred
    p1 = case['parser'](target)
  File "/builds/jvoisin/mat2/libmat2/images.py", line 111, in __init__
    raise ValueError
ValueError
```

Pending on georg/mat2-ci-images#14