core: verification script must return exactly one value

[roman-khimov]

C# node is quite picky as it expects there to be exactly one value returned,
but our testchain actually adds 4 signatures for multisig cases instead of 3
which makes it technically incompatible with C# node.

[codecov]

Codecov Report

Merging #1332 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1332      +/-   ##
==========================================
+ Coverage   68.30%   68.34%   +0.04%     
==========================================
  Files         216      216              
  Lines       18584    18586       +2     
==========================================
+ Hits        12694    12703       +9     
+ Misses       5201     5197       -4     
+ Partials      689      686       -3     

Impacted Files	Coverage Δ
pkg/core/blockchain.go	71.39% <100.00%> (+0.06%)	⬆️
pkg/internal/testchain/address.go	83.33% <100.00%> (ø)
pkg/smartcontract/parameter.go	83.55% <0.00%> (+1.00%)	⬆️
pkg/rpc/client/wsclient.go	84.56% <0.00%> (+1.34%)	⬆️
pkg/core/native/blocked_accounts.go	75.00% <0.00%> (+8.33%)	⬆️

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