core: adjust System.Runtime.GetRandom

[AnnaShaleva]

Close #2518.

[codecov]

Codecov Report

Merging #2519 (8055952) into master (a3285ea) will decrease coverage by 0.02%.
The diff coverage is 76.31%.

@@            Coverage Diff             @@
##           master    #2519      +/-   ##
==========================================
- Coverage   85.13%   85.10%   -0.03%     
==========================================
  Files         294      294              
  Lines       36988    37005      +17     
==========================================
+ Hits        31490    31494       +4     
- Misses       4179     4187       +8     
- Partials     1319     1324       +5     

Impacted Files	Coverage Δ
pkg/config/hardfork_string.go	50.00% <ø> (ø)
pkg/core/interop/context.go	91.89% <ø> (ø)
pkg/core/interops.go	100.00% <ø> (ø)
pkg/core/interop/runtime/util.go	89.02% <52.63%> (-10.98%)	⬇️
pkg/config/hardfork.go	100.00% <100.00%> (ø)
pkg/core/interop_system.go	92.06% <100.00%> (ø)
pkg/rpc/response/result/invoke.go	82.44% <100.00%> (ø)
pkg/core/transaction/transaction.go	84.55% <0.00%> (-1.55%)	⬇️

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