[TestMethod]
    public void TestNextToken()
    {
        string input = "let five = 5;";
        var lexer = new Monkey.Lexer(input);
        Monkey.Token token;
        Monkey.Token[] expectedTokens = { new Monkey.Token(Monkey.Token.LET, "let"),
                                          new Monkey.Token(Monkey.Token.IDENTIFIER, "five"),
                                          new Monkey.Token(Monkey.Token.ASSIGN, "="),
                                          new Monkey.Token(Monkey.Token.INT, "5"),
                                          new Monkey.Token(Monkey.Token.SEMICOLON, ";")
                                        };
        for(int i = 0; i < expectedTokens.Length; i++)
        {
            token = lexer.NextToken();
            if (token.Type != expectedTokens[i].Type)
                Assert.Fail($"Type = {token.Type}, expected Type = {expectedTokens[i].Type}");
            if (token.Literal != expectedTokens[i].Literal)
                Assert.Fail($"Literal = {token.Literal}, expected Literal = {expectedTokens[i].Literal}");
        }
    }