DEPS: Bump testem from 3.18.0 to 3.19.0

Bumps [testem](https://github.com/testem/testem) from 3.18.0 to 3.19.0.
- [Release notes](https://github.com/testem/testem/releases)
- [Commits](testem/testem@v3.18.0...v3.19.0)

---
updated-dependencies:
- dependency-name: testem
  dependency-version: 3.19.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

frontend/discourse/package.json

@@ -150,7 +150,7 @@
     "sinon": "^21.0.3",
     "source-map": "^0.7.6",
     "terser": "^5.46.1",
-    "testem": "^3.16.0",
+    "testem": "^3.19.0",
     "typescript": "^5.9.3",
     "util": "^0.12.5",
     "webpack": "5.99.9",