Addition of unit and integration tests in liquidjava-verifier

[dajh-horta]

Student number: 61835

This pull request adds unit and integration tests to increase coverage of classes in liquidjava-verifier:

Increased coverage for classes:

• LiteralInt;
• LiteralString;
• Ite;
• LJDiagnostic;
• BinaryExpression;
• ConstantFolding;
• ValDerivationNode;
• FunctionInvocation.

Unit tests:

• TestLiteralString: tests LiteralString hashCode() method;
• TestLJDiagnostic: tests LJDiagnostic getTitle() method;
• TestIte: tests Ite hashcode() method.

Integration test:

• TestOptimization: evaluates the interaction between ConstantFolding and BinaryExpression;
• TestFunctionInvocation: tests FunctionInvocation equals() and hashCode() methods with LiteralString args.