Allow asserting on files from the root of the project

[sleberrigaud]

I can't (simply) write assertions on files from the root (basedir) of the project. I have a maven plugin that generates files outside of the target directory. Those generated files are meant to be committed alongside the code in the (git) repository.

I should be able to write an assertion like the following:

assertThat(result).isSuccessful()
                .project()
                .withFile("some-file-from-basedir");

Note that withFile right now, assumes one is looking up files in ${basedir}/target. So changing this would a breaking change.

Right now, I'm working around this by doing something like below:

assertThat(result).isSuccessful()
                .project()
                .withFile("../some-file-from-basedir"); // note the `..`

But even for this, I have to create an empty target directory, as my plugin/test does not do so.

Last note. I've discovered the project today and really like how it makes testing maven plugin finally easy and simple.