More robust testing without a TeX installation

Don't merge this pull request just yet.

 For now, I made the Test checks for requirements more robust to installations where kpsewhich isn't installed.

There is still the task of making sure pathname_find is always used when kpsewhich isn't presented, and a number of the test cases need to explicitly state requirements - e.g. expansion/meaning needs fontenc, which in turn requires t1enc.def installed on the machine. So that needs to be made skippable, somehow.