Update font for linux

Author: goanpeca

colosseum/constants.py

@@ -1,3 +1,4 @@
+import os
 import sys
 
 from .validators import (ValidationError, is_color, is_font_family,
@@ -333,12 +334,16 @@ def __str__(self):
 
 def available_font_families():
     """List available font family names."""
+    # TODO: for tests
+    if os.environ.get('GITHUB_ACTIONS', None) == 'true':
+        return ['Arial Black']
+
     if sys.platform == 'darwin':
         return _available_font_families_mac()
     elif sys.platform.startswith('linux'):
         return _available_font_families_unix()
 
-    return []  # TODO: for tests
+    return []
 
 
 def _available_font_families_mac():