test_config is not parallel-safe

[Xarthisius]

Bug report

Bug summary

During the execution of yt/utilities/tests/test_config.py we modify the actual <XDG_CONFIG_HOME>/yt/yt.toml. This is problematic since tests run in parallel as separate processes and can pick up whatever garbage is being dumped into temporary yt configuration. This can manifest as e.g.:

THIS IS THE REAL CAUSE OF THE FAILURE:
Traceback (most recent call last):
  File "doc/source/cookbook/custom_transfer_function_volume_rendering.py", line 6, in <module>
    ds = yt.load("Enzo_64/DD0043/data0043")
  File "/home/fido/workspace/yt_py38_latest_deps/yt/loaders.py", line 86, in load
    raise FileNotFoundError(msg)
FileNotFoundError: No such file or directory: 'Enzo_64/DD0043/data0043'.

Entire thing went unnoticed cause until yesterday test suite didn't rely on config file. Instead I was doing the following:

 sed -i -e '/test_data_dir/ s:/does/not/exist:/mnt/yt:g' \
    -e '/xray_data_dir/ s:/does/not/exist:/mnt/yt/xray_data:g' \
    -e '/supp_data_dir/ s:/does/not/exist:/mnt/yt/supp_data:g' \
    -e '/__strict_requires/ s/False/True/g' \
    -e '/suppressstreamlogging/ s/False/True/g' yt/config.py

prior to test execution to set whatever is necessary, which is now restored back.

Code for reproduction

$ sudo chown root:root -R $HOME/.config/yt 
$ nosetests -s -vv yt/utilities/tests/test_config.py

Actual outcome

======================================================================
ERROR: test suite for <module 'yt.utilities.tests.test_config' from '/home/xarth/codes/xarthisius/yt/yt/utilities/tests/test_config.py'>
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/xarth/codes/xarthisius/yt/pytest/lib/python3.8/site-packages/nose/suite.py", line 210, in run
    self.setUp()
  File "/home/xarth/codes/xarthisius/yt/pytest/lib/python3.8/site-packages/nose/suite.py", line 293, in setUp
    self.setupContext(ancestor)
  File "/home/xarth/codes/xarthisius/yt/pytest/lib/python3.8/site-packages/nose/suite.py", line 316, in setupContext
    try_run(context, names)
  File "/home/xarth/codes/xarthisius/yt/pytest/lib/python3.8/site-packages/nose/util.py", line 471, in try_run
    return func()
  File "/home/xarth/codes/xarthisius/yt/yt/utilities/tests/test_config.py", line 54, in setUpModule
    os.rename(cfgfile, cfgfile + ".bak_test")
PermissionError: [Errno 13] Permission denied: '/home/xarth/.config/yt/yt.toml' -> '/home/xarth/.config/yt/yt.toml.bak_test'

Expected outcome

Config not modified.