Add file helpers to the InvokeMainTestCase framework

[DavidWhitlock]

I've noticed that in the text file and pretty print projects students aren't writing integration tests. I should add some file helpers to the InvokeMainTestCase framework.

[DavidWhitlock]

One file helper that would be helpful is a method that copies a resource file to a file in a directory (such as the JUnit @TempDir directory).

[DavidWhitlock]

Oh. And it's possible that failsafe isn't configured to mark src/it/resources as a resource directory.

[DavidWhitlock]

Another helpful method would read the contents of a text file into a String.