add tests for liealgebras/type_A

Hi Mary, 

I've added a couple of files to your work.  

The first is `__init__.py`.  You can safely ignore this.

The second is a testing file.  You should pay attention to this one and add to it as you develop your code.  You can run tests in your directory by running the command

    bin/test sympy/liealgebras

From the main sympy directory.  If you run this now it will show that you have an error in roots.  You can fix this error, run the tests again, see if it works.  If the tests pass then you should commit your work and push up to github.  I recommend that you add more tests for your other methods.  It's a great way to make consistent progress.

This pull request is to your branch.  You'll have to decide if you want to merge it or not.  If you do want to merge it there should be a green button somewhere on the bottom of the page.  Note that this will merge my added files into your repository on github.  You'll have to pull these changes down to your computer before you can run bin/test