Move tests into main JastAdd2 repository

[Jesper Öqvist]

Currently the tests live in a separate repository which is linked as a Git submodule into the jastadd2 repository. This presents some problems when fixing issues in separate branches, the main one being that any tests for a fixed issue needs to be first merged into the test repository before a bugfix branch can be merged into jastadd2.

It would significantly simplify the workflow when fixing bugs in branches and merging through pull requests if all tests were just in the main jastadd2 repository. That way the bugfix and tests would be tied together more naturally and could be merged in one single merge.