Diff output in Interactive tests

[thepieterdc]

This PR adds the possibility to use diff feedback when using an Interactive test.

Usage:

public class InteractiveTest {
    @Rule
    public final Interactive interactive = Interactive.forClass(SomeExercise.class);

    @Test
    public void testInvalidOp() throws Throwable {
        final String expected = "Expected output from student.";

        this.interactive
            // Feed some input text.
            .feedLines("Input string", "Some other input")
            // Run the program.
            .callMain()
            // Check the output.
            .assertOutput(expected);
    }
}

Other notable changes

• The feedLine methods are deprecated and renamed to feedLines, but are not yet removed to ensure no existing exercises are broken.
• It is now possible to use command-line arguments when calling the main function, by using callMain("argument1", "argument2", ...)

[thepieterdc]

Also, you accidently committed a class file in the testing directory.

Not sure how that can occur but I will remove it.