add some unit test for sentinel-transport-netty-http module

[cdfive]

Describe what this PR does / why we need it

The sentinel-transport-netty-http module doesn't have test package, add some unit test.

Does this pull request fix one issue?

NONE

Describe how you did it

Import junit and mockito-core dependency, and use Netty's EmbeddedChannel for test ChannelHandler.

Describe how to verify it

run mvn test

Special notes for reviews

one test case testGetRuleCommandFlowSomeRules, since I haven't found a good way yet，comment the @Test annotation and marked it with the FIXME comment, for improving it in the future.

[codecov-io]

Codecov Report

Merging #321 into master will decrease coverage by 0.43%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #321      +/-   ##
============================================
- Coverage     38.71%   38.28%   -0.44%     
+ Complexity      936      930       -6     
============================================
  Files           218      218              
  Lines          6912     6912              
  Branches        938      938              
============================================
- Hits           2676     2646      -30     
- Misses         3897     3932      +35     
+ Partials        339      334       -5

Impacted Files	Coverage Δ	Complexity Δ
...a/csp/sentinel/slots/statistic/base/Striped64.java	26.04% <0%> (-23.96%)	5% <0%> (-2%)
...a/csp/sentinel/slots/statistic/base/LongAdder.java	17.02% <0%> (-14.9%)	4% <0%> (-4%)

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[sczyh30]

LGTM

[sczyh30]

Great! Thanks for contributing!