build(node): don't throw on deprecation in unit tests (googleapis#846)

Fixes googleapis#1134 🦕

Removes the commit body and relative PR# from the commit message.

For example, for this commit: googleapis/synthtool@9763f20

`post-processor-changes.txt` would contain

```
build: enable npm for php/python builds

Source-Link: googleapis/synthtool@9763f20
```

instead of

```
build: enable npm for php/python builds (googleapis#1133)

* build: enable npm for php/python builds

* update comment
Source-Link: googleapis/synthtool@9763f20
```
Source-Link: googleapis/synthtool@e934b93
Post-Processor: gcr.io/repo-automation-bots/owlbot-nodejs:latest@sha256:805e2e389eafefa5ed484c30b83a7a875e6b1c7ee125d812e8b01ecc531c3fac

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/repo-automation-bots/owlbot-nodejs:latest
-  digest: sha256:c9c7828c165b1985579098978877935ee52dda2b1b538087514fd24fa2443e7a
+  digest: sha256:805e2e389eafefa5ed484c30b83a7a875e6b1c7ee125d812e8b01ecc531c3fac

.github/workflows/ci.yaml

@@ -24,6 +24,8 @@ jobs:
       - run: rm -rf node_modules
       - run: npm install
       - run: npm test
+        env:
+          MOCHA_THROW_DEPRECATION: false
   windows:
     runs-on: windows-latest
     steps:
@@ -33,6 +35,8 @@ jobs:
           node-version: 14
       - run: npm install
       - run: npm test
+        env:
+          MOCHA_THROW_DEPRECATION: false
   lint:
     runs-on: ubuntu-latest
     steps: