chore(node): exclude test fixtures from post-processing (#3909)

Excludes `core/generator/gapic-generator-typescript/test-fixtures`
from directory traversal in `node_mono_repo.py` to prevent the
post-processor from re-adding them to `release-please-config.json`.

See googleapis/google-cloud-node#8171 for
an example of the issue.

Source-Link: googleapis/synthtool@e80fef9
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest@sha256:4fe73296173f9cbfced42915365a33981ee6a291dd2dc0edb1a0a2e3abeeeed8

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,5 +13,5 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest
-  digest: sha256:5ef945dd0b346b78caabeaf973467273a31394c074f5ec3f85cb66769036bab5
-# created: 2026-02-12T15:40:42.386984339Z
+  digest: sha256:4fe73296173f9cbfced42915365a33981ee6a291dd2dc0edb1a0a2e3abeeeed8
+# created: 2026-05-06T03:27:26.255454877Z