Fixture `non_mapping_dict_subclass` is wrongly named
#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution.]

In `conftest.py`, a fixture is defined with the name `non_mapping_dict_subclass`, which returns a minimal class implementing the `collections.abc.Mapping` base class by minimally wrapping a `dict`.

Obviously, the intended name was `non_dict_mapping_subclass`, which also is the name of the class returned (casing apart).

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 1d13390e26bc7b1f5fb597bc5df1591f5d160e1f
python           : 3.6.10.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.3.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_GB.UTF-8
pandas           : 1.0.3
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.0.0.post20200309
Cython           : 0.29.15
pytest           : 5.4.1
hypothesis       : 5.7.0
sphinx           : 2.4.4
blosc            : 1.8.3
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : 4.3.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.2.1
fastparquet      : 0.3.3
gcsfs            : None
lxml.etree       : 4.3.1
matplotlib       : 3.2.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.1
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : 5.4.1
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.4.1
sqlalchemy       : 1.3.15
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : 0.15.0
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.8
numba            : 0.48.0
</details>
