Fix flaky TestVMessDynamicPort (#723)

[yuhan6665]

XTLS/Xray-core#723

[codecov-commenter]

Codecov Report

Merging #1289 (1cba3d0) into master (c278db5) will increase coverage by 0.02%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #1289      +/-   ##
==========================================
+ Coverage   44.64%   44.67%   +0.02%     
==========================================
  Files         485      485              
  Lines       29524    29538      +14     
==========================================
+ Hits        13182    13195      +13     
- Misses      14939    14940       +1     
  Partials     1403     1403              

Impacted Files	Coverage Δ
testing/scenarios/common.go	77.69% <92.85%> (+3.55%)	⬆️
common/drain/drainer.go	65.51% <0.00%> (-6.90%)	⬇️
transport/pipe/impl.go	87.87% <0.00%> (-2.03%)	⬇️
transport/internet/kcp/connection.go	72.37% <0.00%> (+0.55%)	⬆️

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[kslr]

Will anyone be using Dynamic Port? How about we delete it?

[yuhan6665]

I have no idea about that. Just fixing some tests :)

[xiaokangwang]

I think this function is not being widely used right now, but until it cause any issues we can keep it.

I believe this pull request is ready to be merged.