Bump coverage from 7.6.7 to 7.6.8 in /testbed

Bumps [coverage](https://github.com/nedbat/coveragepy) from 7.6.7 to 7.6.8.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](nedbat/coveragepy@7.6.7...7.6.8)

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

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

testbed/pyproject.toml

@@ -4,7 +4,7 @@ version = "0.0.1"
 
 [project.optional-dependencies]
 test = [
-    "coverage==7.6.7",
+    "coverage==7.6.8",
     "coverage-conditional-plugin == 0.9.0",
     # fonttools is only needed by Android, but we need to use
     # sys.platform == 'linux' as there's no dependency identifier