This change disables warnings from pytest. The warnings results in
verbose output from `check()` including a list of test names in the
logs. After disabling the warnings, the last 500 KiB of the logs
as shown in the GitLab web UI becomes useful again and will include the
details of failing tests.

This change also disables a test that fails intermittently.

The bean-file command which is a part of this package uses pytest at run
time. It is used indirectly through
<https://bitbucket.org/blais/beancount/src/default/beancount/ingest/__init__.py>
which is installed to
`usr/lib/python3.8/site-packages/beancount/ingest/__init__.py` and uses
pytest at line 13.

Before this change:

1. Install beancount: `sudo apk add beancount`
2. Check that `py3-pytest` is not installed: `apk list --installed
   '*pytest*'`
3. Running `/usr/bin/bean-file` gives the output below:

```
Traceback (most recent call last):
  File "/usr/bin/bean-file", line 4, in <module>
    from beancount.ingest.file import main; main()
  File "/usr/lib/python3.8/site-packages/beancount/ingest/__init__.py", line 13, in <module>
    import pytest
ModuleNotFoundError: No module named 'pytest'
```

After this change `/usr/bin/bean-file` behaves as expected.

Disable one additional test that fails on Python 3.8

Closes GH-7749