integration: 调整主从的集成测试

[flycash]

• 给 roundrobin 的 NewSlaves 增加了 error 返回值（以防万一）
• 调整了部分局部变量命名
• 引入了 testSlaves 来替代已有的双层结构

[codecov-commenter]

Codecov Report

Merging #163 (8634707) into dev (2b3a9b3) will increase coverage by 0.04%.
The diff coverage is 88.88%.

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@@            Coverage Diff             @@
##              dev     #163      +/-   ##
==========================================
+ Coverage   79.64%   79.69%   +0.04%     
==========================================
  Files          29       30       +1     
  Lines        2334     2354      +20     
==========================================
+ Hits         1859     1876      +17     
- Misses        392      394       +2     
- Partials       83       84       +1     

Impacted Files	Coverage Δ
master_slave_db.go	88.00% <ø> (ø)
internal/slaves/dns/dns.go	78.21% <66.66%> (-0.22%)	⬇️
internal/slaves/roundrobin/roundrobin.go	100.00% <100.00%> (ø)

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