Fix some tests to use udp.PickPort()

[yuhan6665]

XTLS/Xray-core#739

[codecov-commenter]

Codecov Report

Merging #1302 (418b499) into master (deb9d08) will decrease coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1302      +/-   ##
==========================================
- Coverage   44.64%   44.40%   -0.24%     
==========================================
  Files         485      476       -9     
  Lines       29534    29435      -99     
==========================================
- Hits        13184    13072     -112     
- Misses      14945    14949       +4     
- Partials     1405     1414       +9     

Impacted Files	Coverage Δ
transport/internet/udp/dispatcher.go	64.76% <0.00%> (-12.39%)	⬇️
common/buf/reader.go	82.35% <0.00%> (-5.89%)	⬇️
testing/servers/tcp/tcp.go	81.81% <0.00%> (-5.46%)	⬇️
transport/internet/udp/hub.go	73.61% <0.00%> (-4.17%)	⬇️
proxy/freedom/freedom.go	45.00% <0.00%> (-4.00%)	⬇️
transport/internet/system_listener.go	50.00% <0.00%> (-4.00%)	⬇️
transport/internet/kcp/dialer.go	80.39% <0.00%> (-3.93%)	⬇️
testing/scenarios/common.go	70.68% <0.00%> (-3.45%)	⬇️
transport/internet/kcp/receiving.go	97.31% <0.00%> (-2.69%)	⬇️
transport/internet/quic/hub.go	72.00% <0.00%> (-2.67%)	⬇️
... and 14 more

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[yuhan6665]

This check fail:
https://github.com/v2fly/v2ray-core/pull/1302/checks?check_run_id=3726781714#step:5:2727
is not the original issue but a different one. This one seems more complicated which I will work on the fix later ;)

[xiaokangwang]

Thanks for your contribution.

@yuhan6665 did't describe the root cause of the issue being fixed. Based on the content of the pull request I have a educated guess of why this fix is needed.

In the previous version of the test code, when attempting to listen for a UDP socket, the port availability check check for TCP port instead of UDP port. This make it possible to select a UDP port that is unavailable.

This pull request is ready to be merged.

[yuhan6665]

Thanks for the review!
I will try to improve my English description in the future ;)