Add trie node unit tests

[blukat29]

Proposed changes

• Add unit tests
  • storage/statedb/node_test.go tests decodeNode()
  • storage/statedb/hasher_test.go tests hasher.hash()
• Minor syntax improvement
  • append(valueNode{}, val...) -> valueNode(val)
• Silence print_trie_test.go

Types of changes

• Bugfix
• New feature or enhancement
• Others

Checklist

• I have read the CONTRIBUTING GUIDELINES doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

Further comments

[jeongkyun-oh]

LGTM except for minors