Maintenance: Update dependency test-unit to v3.6.7

Gemfile.lock

@@ -675,7 +675,7 @@ GEM
     telephone_number (1.4.22)
     terser (1.2.4)
       execjs (>= 0.3.0, < 3)
-    test-unit (3.6.5)
+    test-unit (3.6.7)
       power_assert
     thor (1.3.2)
     thread_safe (0.3.6)