##### SUMMARY
When running `ansible-test units ...`, failed tests have no diagnostic messages attached to the assertion error.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
ansible-test

##### ANSIBLE VERSION
```paste below
ansible 2.9.5
  config file = /etc/ansible/ansible.cfg
  configured module search path = ['/home/tadej/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/tadej/xlab/ansible/ansible_collections/debug/collection/venv/lib/python3.7/site-packages/ansible
  executable location = /home/tadej/xlab/ansible/ansible_collections/debug/collection/venv/bin/ansible
  python version = 3.7.6 (default, Jan 30 2020, 09:44:41) [GCC 9.2.1 20190827 (Red Hat 9.2.1-1)]
```

##### CONFIGURATION
`ansible-config dump --only-changed` reported no changes.

##### OS / ENVIRONMENT
Tested on latest stable Fedora, Ansible was installed in a virtual environment using pip.

##### STEPS TO REPRODUCE
Run `ansible-test units path/to/test_file.py`, where *path/to/test_file.py* should have a failing test case akin to the next one:

    def test_x():
        assert dict(a=1) == dict(a=2)

See https://github.com/tadeboro/ansible-test-bad-assert for ready-to-run example.

##### EXPECTED RESULTS
In the case above, `pytest` prints this out:
```
========================== FAILURES ===========================
___________________________ test_x ____________________________

    def test_x():
>       assert dict(a=1) == dict(a=2)
E       AssertionError: assert {'a': 1} == {'a': 2}
E         Differing items:
E         {'a': 1} != {'a': 2}
E         Use -v to get the full diff

tests/unit/test_x.py:2: AssertionError
```


##### ACTUAL RESULTS
`ansible-test units` prints this:
```
========================== FAILURES ===========================
___________________________ test_x ____________________________
[gw1] linux -- Python 3.7.6 /tmp/python-hvsrfqdb-ansible/python
def test_x():
>       assert dict(a=1) == dict(a=2)
E       AssertionError

tests/unit/test_x.py:2: AssertionError
```

##### ADDITIONAL NOTES

I can bring back the diagnostic messages by commenting out the line https://github.com/ansible/ansible/blob/35996e57abba1f40abb493379d5cbe2dd90e65c3/test/lib/ansible_test/_data/pytest/plugins/ansible_pytest_collections.py#L40, but this of course then breaks other stuff.
"""Enable unit testing of Ansible collections. PYTEST_DONT_REWRITE"""
from __future__ import (absolute_import, division, print_function)
__metaclass__ = type
import os
ANSIBLE_COLLECTIONS_PATH = os.path.join(os.environ['ANSIBLE_COLLECTIONS_PATH'], 'ansible_collections')
ANSIBLE_CONTROLLER_MIN_PYTHON_VERSION = tuple(int(x) for x in os.environ['ANSIBLE_CONTROLLER_MIN_PYTHON_VERSION'].split('.'))
def collection_resolve_package_path(path):
    """Configure the Python package path so that pytest can find our collections."""
    for parent in path.parents:
        if str(parent) == ANSIBLE_COLLECTIONS_PATH:
            return parent
    raise Exception('File "%s" not found in collection path "%s".' % (path, ANSIBLE_COLLECTIONS_PATH))
def collection_pypkgpath(self):
    """Configure the Python package path so that pytest can find our collections."""
    for parent in self.parts(reverse=True):
        if str(parent) == ANSIBLE_COLLECTIONS_PATH:
            return parent
    raise Exception('File "%s" not found in collection path "%s".' % (self.strpath, ANSIBLE_COLLECTIONS_PATH))
def pytest_configure():