testing: lookup in no-GOPATH locations for packages in gocodec testing

[tsachiherman]

Summary

lookup in no-GOPATH locations for packages in gocodec testing; The existing code is trying to use the GOPATH as the base for the packages directories, however, this might not be the case when tests are being executed on travis.

Test Plan

This is a test.

[codecov-commenter]

Codecov Report

Merging #2562 (1d35dfa) into master (01248d2) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2562      +/-   ##
==========================================
+ Coverage   46.89%   46.92%   +0.03%     
==========================================
  Files         346      346              
  Lines       55616    55630      +14     
==========================================
+ Hits        26081    26106      +25     
+ Misses      26580    26576       -4     
+ Partials     2955     2948       -7     

Impacted Files	Coverage Δ
protocol/codec_tester.go	0.00% <0.00%> (ø)
ledger/catchpointwriter.go	68.18% <0.00%> (-1.30%)	⬇️
data/transactions/verify/txn.go	46.39% <0.00%> (-1.04%)	⬇️
ledger/acctupdates.go	61.83% <0.00%> (-0.09%)	⬇️
network/wsNetwork.go	61.02% <0.00%> (+0.28%)	⬆️
ledger/eval.go	76.68% <0.00%> (+0.29%)	⬆️
catchup/peerSelector.go	100.00% <0.00%> (+1.04%)	⬆️
ledger/blockqueue.go	82.18% <0.00%> (+1.14%)	⬆️
crypto/merkletrie/node.go	93.48% <0.00%> (+1.86%)	⬆️
crypto/merkletrie/trie.go	68.61% <0.00%> (+2.18%)	⬆️
... and 2 more

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[ghost]

looks good, had an optional nit