misc,ext: Update pre-commit hook to run on ext/testlib

Though in ext, we regularly modify these files to add features and
extend our testlib testing infrastructure. Ergo, the pre-commit checks
should be run.

Change-Id: I921a263f25f850b03e5535a8a1f509921c124763

.pre-commit-config.yaml

@@ -40,7 +40,7 @@ default_language_version:
 
 exclude: |
   (?x)^(
-    ext/.*|
+    ext/(?!testlib/).*|
     build/.*|
     src/systemc/ext/.*|
     src/systemc/tests/.*/.*|