Improve code coverage

[kostko]

See #1134.

[codecov]

Codecov Report

Merging #2832 into master will increase coverage by 1.23%.
The diff coverage is 77.46%.

@@            Coverage Diff             @@
##           master    #2832      +/-   ##
==========================================
+ Coverage   66.25%   67.49%   +1.23%     
==========================================
  Files         345      344       -1     
  Lines       33269    33071     -198     
==========================================
+ Hits        22044    22321     +277     
+ Misses       8361     7895     -466     
+ Partials     2864     2855       -9     

Impacted Files	Coverage Δ
go/common/address.go	72.60% <ø> (+9.93%)	⬆️
go/oasis-node/cmd/root.go	86.48% <ø> (-0.36%)	⬇️
go/storage/mkvs/tree.go	92.85% <ø> (+10.09%)	⬆️
go/oasis-node/cmd/ias/auth_registry.go	69.23% <65.90%> (+69.23%)	⬆️
go/common/cbor/codec.go	77.14% <80.00%> (+12.43%)	⬆️
go/common/args.go	100.00% <100.00%> (ø)
go/common/testing/e2e_coverage.go	100.00% <100.00%> (ø)
go/runtime/client/tests/tester.go	100.00% <100.00%> (ø)
go/consensus/tendermint/api/errors.go	86.66% <0.00%> (-13.34%)	⬇️
go/consensus/tendermint/roothash/roothash.go	65.54% <0.00%> (-6.00%)	⬇️
... and 55 more

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