HexStringInterpreter in Testing utils may accept non-hexadecimal char

[Hecate2]

neo-devpack-dotnet/src/Neo.SmartContract.Testing/Interpreters/HexStringInterpreter.cs

Line 9 in 028445c

public static readonly Regex HexRegex = new(@"^[a-zA-Z0-9_]+$");

It should only take a-fA-F, though it does not really matter. The current test system works with bugs (as features).
A larger problem (handled in #1265 ) is that if (instruction.Operand.Span.TryGetString(out var str) makes the tests slow. I am writing a PR to accelerate tests.