Create a dedicated lexicon for testing

[Kaljurand]

Currently many test cases that are run by the scripts in the tests-directory depend on a lexicon that is larger than APE's built-in/demo lexicon. The solution is to download https://github.com/Attempto/Clex before the tests are run, but this has the following shortcomings:

• dependency on an external resource
• slowness
• confusing code

It would be better to include a dedicated lexicon for the test run in the tests-directory. The simplest is just to include Clex, but better would be to drop all entries that are not used by any tests, and add some entries that would test some aspects of the lexicon format.