Exclude the test files from pre-commit.

They're not meant to be checked by the hooks in .pre-commit-config.yaml
(that test the code in the repo) but by the ones in
.pre-commit-hooks.yaml (test cases for the hooks in the repo).

.pre-commit-config.yaml

 ---
+exclude: test_files
 repos:
   - repo: https://github.com/pre-commit/pre-commit-hooks.git
     rev: v4.0.1
@@ -10,7 +11,7 @@ repos:
       - id: check-yaml
       - id: detect-private-key
       - id: end-of-file-fixer
-        exclude: VERSION|test_files
+        exclude: VERSION
       - id: trailing-whitespace
 
   - repo: https://github.com/codespell-project/codespell.git