Create testing setup

[JonasAlaif]

Run the tests with (you may need to run ./x.py build beforehand):

./x.py test -p prusti-tests &> test.txt

Currently I get:

test result: FAIL. 163 failed; 118 passed

---

In your own branch, use to get the testing setup:

git cherry-pick e952f5d0f46eb9e9dee7f24013d392b2feb856bf

And then (if it's the last commit) to undo it, use:

git reset --hard HEAD~1

[Aurel300]

(merged as part of #106)

[JonasAlaif]

As mentioned in #108 let's not merge this :) we can take some parts of it as needed if we want