tool/lib/minitest/unit.rb: add "omit" as an alias to "skip"

According to rdoc, test-unit provides omit instead of skip.
This is a compatibility layer to make it work with both test-unit and
tool/lib/minitest.

tool/lib/minitest/unit.rb

@@ -718,6 +718,8 @@ module MiniTest
       raise MiniTest::Skip, msg, bt
     end
 
+    alias omit skip
+
     ##
     # Was this testcase skipped? Meant for #teardown.