test: Use gomock instead of implement interfaces manually

[helayoty]

Using gomock would be much easier than implementing interfaces manually in the network and auth cases.

Steps to re-generate the mock (no need to rerun it as long as interfaces are not changed):

go install github.com/golang/mock/mockgen@v1.6.0

cd pkg/network
mockgen -source=aci_network.go -destination=mock_aci_network_test.go -package=network

cd ../auth
mockgen -source=auth_config.go -destination=mock_auth_config_test.go -package=auth

[codecov-commenter]

Codecov Report

Merging #530 (86842ab) into master (59fd228) will increase coverage by 0.78%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master     #530      +/-   ##
==========================================
+ Coverage   60.99%   61.78%   +0.78%     
==========================================
  Files          17       15       -2     
  Lines        2505     2473      -32     
==========================================
  Hits         1528     1528              
+ Misses        865      833      -32     
  Partials      112      112              

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