tests: Delete env vars that could interfere

Several environment variables can influence which config file yamllint
chooses to use [1]. Before this change, if you set those environment
variables and ran “python -m unittest discover”, then you could cause
certain tests to use the wrong config file and fail.

Fixes #621.

[1]: <152ba20f33/yamllint/cli.py (L180-L188)>

tests/test_cli.py

@@ -66,6 +66,26 @@ def utf8_available():
         return False
 
 
+def setUpModule():
+    # yamllint uses these environment variables to find a config file.
+    env_vars_that_could_interfere = (
+        'YAMLLINT_CONFIG_FILE',
+        'XDG_CONFIG_HOME',
+        # These variables are used to determine where the user’s home
+        # directory is. See
+        # https://docs.python.org/3/library/os.path.html#os.path.expanduser
+        'HOME',
+        'USERPROFILE',
+        'HOMEPATH',
+        'HOMEDRIVE'
+    )
+    for name in env_vars_that_could_interfere:
+        try:
+            del os.environ[name]
+        except KeyError:
+            pass
+
+
 class CommandLineTestCase(unittest.TestCase):
     @classmethod
     def setUpClass(cls):