fix: requirements-test.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-FONTTOOLS-6133203
- https://snyk.io/vuln/SNYK-PYTHON-NUMPY-2321964
- https://snyk.io/vuln/SNYK-PYTHON-NUMPY-2321966
- https://snyk.io/vuln/SNYK-PYTHON-NUMPY-2321970
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-5918878
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-6043904
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-6182918
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-6219984
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-6219986

Author: snyk-bot

requirements-test.txt

@@ -11,3 +11,6 @@ seaborn
 xarray>=0.13
 wheel
 pyarrow
+fonttools>=4.43.0 # not directly required, pinned by Snyk to avoid a vulnerability
+numpy>=1.22.2 # not directly required, pinned by Snyk to avoid a vulnerability
+pillow>=10.2.0 # not directly required, pinned by Snyk to avoid a vulnerability