Update test-unit to version 3.6.8

Author: depfu[bot]

Gemfile.lock

@@ -393,7 +393,7 @@ GEM
     stringio (3.1.6)
     terminal-table (4.0.0)
       unicode-display_width (>= 1.1.1, < 4)
-    test-unit (3.6.7)
+    test-unit (3.6.8)
       power_assert
     thor (1.3.2)
     timeout (0.4.3)