TestConf: Use registry to discover test network

[mkykadir]

also reduce the discover interval.

Fixes #3033

[codecov[bot]]

Codecov Report

Merging #3286 (a498f97) into v0.x.x (f7ccda5) will increase coverage by 1.18%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           v0.x.x    #3286      +/-   ##
==========================================
+ Coverage   87.92%   89.11%   +1.18%     
==========================================
  Files         164      165       +1     
  Lines       16951    17060     +109     
==========================================
+ Hits        14905    15203     +298     
+ Misses       2046     1857     -189     

Flag	Coverage Δ
integration	45.18% <ø> (?)
unittests	87.87% <100.00%> (-0.06%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/test/Base.d	80.47% <ø> (-0.20%)	:arrow_down:
source/agora/test/GossipProtocol.d	93.93% <100.00%> (ø)
source/agora/network/Client.d	88.23% <0.00%> (-5.05%)	:arrow_down:
source/agora/consensus/protocol/Nominator.d	91.45% <0.00%> (-0.54%)	:arrow_down:
source/agora/consensus/state/Ledger.d	90.39% <0.00%> (-0.34%)	:arrow_down:
source/agora/network/Manager.d	78.48% <0.00%> (ø)
source/agora/node/main.d	44.85% <0.00%> (ø)
source/agora/node/Validator.d	92.46% <0.00%> (+0.03%)	:arrow_up:
source/agora/common/BanManager.d	87.39% <0.00%> (+0.84%)	:arrow_up:
... and 11 more

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