chore(deps-dev): bump testem from 3.15.1 to 3.15.2

Bumps [testem](https://github.com/testem/testem) from 3.15.1 to 3.15.2.
- [Release notes](https://github.com/testem/testem/releases)
- [Commits](testem/testem@v3.15.1...v3.15.2)

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

Signed-off-by: dependabot[bot] <[email protected]>

package.json

@@ -182,7 +182,7 @@
     "sanctuary-show": "3.0.0",
     "sinon": "=18.0.1",
     "terser-webpack-plugin": "5.3.10",
-    "testem": "=3.15.1",
+    "testem": "=3.15.2",
     "typescript": "=5.6.2",
     "webpack": "=5.94.0",
     "webpack-cli": "5.1.4",