Bump fonttools from 4.55.5 to 4.55.6 in /testbed

Bumps [fonttools](https://github.com/fonttools/fonttools) from 4.55.5 to 4.55.6.
- [Release notes](https://github.com/fonttools/fonttools/releases)
- [Changelog](https://github.com/fonttools/fonttools/blob/main/NEWS.rst)
- [Commits](fonttools/fonttools@4.55.5...4.55.6)

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

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

testbed/pyproject.toml

@@ -9,7 +9,7 @@ test = [
     # fonttools is only needed by Android, but we need to use
     # sys.platform == 'linux' as there's no dependency identifier
     # that can target Android exclusively until 3.13 lands.
-    "fonttools==4.55.5 ; sys.platform == 'linux'",
+    "fonttools==4.55.6 ; sys.platform == 'linux'",
     "pillow==11.0.0",
     "pytest==8.3.4",
     "pytest-asyncio==0.25.2",