Cleanup spec tests

[jtraglia]

• Use more testify.
• Remove HexStr from struct fields.
  • I don't think it's very necessary.
• Remove assertTestCaseValid function.
  • Replaced with require.True(t, testCaseValid).
• Rename ProofIsValidPredicate to ProofIsValid.
  • I don't think "predicate" is that valuable.
• Add hexStrToProof function.
  • I think this is clearer than hexStrToCommitment + cast.