Report encoding error in test_add_unicode as error

This makes the test explicitly error out, rather than skipping, if
it appears the environment doesn't support encoding Unicode
filenames. Platforms these days should be capable of that, and
reporting it as an error lessens the risk of missing a bug in the
test code (that method or a fixture) if one is ever introduced.
Erroring out will also make it easier to see the details in the
chained UnicodeDecodeError exception.

This does not affect the behavior of GitPython itself. It only
changes how a test reports an unusual condition that keeps the test\
from being usefully run.

Author: EliahKagan

test/test_base.py

@@ -7,7 +7,7 @@
 import os
 import sys
 import tempfile
-from unittest import SkipTest, skipIf
+from unittest import skipIf
 
 from git import Repo
 from git.objects import Blob, Tree, Commit, TagObject
@@ -126,7 +126,7 @@ def test_add_unicode(self, rw_repo):
         try:
             file_path.encode(sys.getfilesystemencoding())
         except UnicodeEncodeError as e:
-            raise SkipTest("Environment doesn't support unicode filenames") from e
+            raise RuntimeError("Environment doesn't support unicode filenames") from e
 
         with open(file_path, "wb") as fp:
             fp.write(b"something")