Enhance the test coverage part-8 : dubbo-filter module

[lovepoem]

Add unit test to dubbo-filter module

[qinnnyul]

@lovepoem hey, I would like to increase test coverages for dubbo-filter module.by the way, what is the target goal? 80%? or something else?

[lovepoem]

ok，the base goal：60%，you can see in the page： https://github.com/apache/incubator-dubbo/wiki/Test-coverage-guide。

The more enough,the better

[htynkn]

PR #1715 add unit test for dubbo-filter-cache. we still need unit test for dubbo-filter-validation

[qinnnyul]

good eye! I will work on it, thanks

[lovepoem]

close via #1715 and #1736