Functional interfaces verification

[alisevych]

Description

Currently UTBot verifies only the state of the resulting Objects.
When a functional interface is returned there should be some verification of it.
At the moment there are only expected and actual - of Object type - without assertions: see bug #1034

Idea

Add verification of returned functional interface's behavior.
Generate some value to input to its method and compare with the expected value.
Fuzzing can be used for that.

Use cases

Functional interfaces returned from a method.

Potential alternatives

Check the Class of the returned functional interface.

Example

Generate tests for the following code:

    Function<String, Integer> returnLambda(int buffer) {
        return s -> s.length() + buffer;
    }

Expected result

    ///region FUZZER: SUCCESSFUL EXECUTIONS for method returnLambda(int)

    public void testReturnLambda() throws Throwable {
        Lambda lambda = new Lambda();

        Function actual = lambda.returnLambda(1);
        
        assertEquals(actual.apply("abyrvalg"), 9);
    }
    ///endregion

Actual result

    ///region FUZZER: SUCCESSFUL EXECUTIONS for method returnLambda(int)

    public void testReturnLambda() throws Throwable {
        Lambda lambda = new Lambda();

        Function actual = lambda.returnLambda(1);

        Function expected = ((Function) buildStaticLambda(Function.class, Lambda.class, "<FAKE>"));
    }
    ///endregion

[alisevych]

@Damtev Please add your concerns