build(deps-dev): bump testem from 2.15.0 to 2.15.1
build(deps-dev): bump testem from 2.15.0 to 2.15.1

Bumps [testem](https://github.com/testem/testem) from 2.15.0 to 2.15.1.
- [Release notes](https://github.com/testem/testem/releases)
- [Changelog](https://github.com/testem/testem/blob/master/CHANGELOG.md)
- [Commits](https://github.com/testem/testem/compare/v2.15.0...v2.15.1)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
rwjblue/ember-template-lint