Bump straxen from 2.1.3 to 2.1.5 in /extra_requirements

Bumps [straxen](https://github.com/XENONnT/straxen) from 2.1.3 to 2.1.5.
- [Release notes](https://github.com/XENONnT/straxen/releases)
- [Changelog](https://github.com/XENONnT/straxen/blob/master/HISTORY.md)
- [Commits](XENONnT/straxen@v2.1.3...v2.1.5)

---
updated-dependencies:
- dependency-name: straxen
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

extra_requirements/requirements-tests.txt

@@ -1,4 +1,4 @@
 strax==1.5.4
-straxen==2.1.3
+straxen==2.1.5
 epix==0.3.5
 git+https://github.com/XENONnT/ax_env