Tests: Support e2e-go tests when repo is not installed in GOPATH.

[winder]

Summary

Running e2e go tests fails when the repo is not cloned inside the GOPATH. This change looks up the current directory using runtime.Caller as an alternative to hard coding the GOPATH location.

note: the Makefile also solves this problem by setting the TESTDATADIR environment variable, but that doesn't work when running tests from an IDE.

Test Plan

Existing tests cover this code path. Additionally I manually verified by running a number of e2e-go/expect tests locally

[codecov-commenter]

Codecov Report

Merging #3949 (a00b0c2) into master (f83e312) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3949      +/-   ##
==========================================
- Coverage   49.83%   49.81%   -0.03%     
==========================================
  Files         409      409              
  Lines       68897    68897              
==========================================
- Hits        34337    34318      -19     
- Misses      30865    30873       +8     
- Partials     3695     3706      +11     

Impacted Files	Coverage Δ
network/netprio.go	69.56% <0.00%> (-8.70%)	⬇️
ledger/tracker.go	70.56% <0.00%> (-6.07%)	⬇️
cmd/algoh/blockWatcher.go	77.77% <0.00%> (-3.18%)	⬇️
network/wsPeer.go	68.61% <0.00%> (-2.23%)	⬇️
data/abi/abi_type.go	87.67% <0.00%> (-0.95%)	⬇️
ledger/acctupdates.go	69.43% <0.00%> (-0.14%)	⬇️
node/node.go	25.03% <0.00%> (+1.84%)	⬆️

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