[PIP] Update pytest-subtests requirement

Updates the requirements on [pytest-subtests](https://github.com/pytest-dev/pytest-subtests) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest-subtests/releases)
- [Changelog](https://github.com/pytest-dev/pytest-subtests/blob/main/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-subtests@v0.12.0...v0.15.0)

---
updated-dependencies:
- dependency-name: pytest-subtests
  dependency-version: 0.15.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

test_requirements.txt

@@ -1,6 +1,6 @@
 # Test requirements
 pytest>=7.0.0,<8.5
-pytest-subtests>=0.12.0,<0.15
+pytest-subtests>=0.12.0,<0.16
 python-gnupg>=0.5.0,<0.6
 trustme>=1.1.0,<1.3
 jinja2>=3.1.4,<3.2