Implement testing "framework"

[akerbos]

We need some form of automated test.

Create a file with commands and expected result (i.e. log messages). Check which log sections are there (i.e. which engines/extensions ran) and how many (which?) messages of which type are included.

Depends on #103

[reitzig]

Thoughts:

• Specify tests in a structured way. (Ruby? JSON with schema?)
  - which test document (if any)
  - which parameters (potentially multiple sets?)
  - the expected behaviour: which engines and extensions are run how often and when?
  - the expected output: number of errors and warnings; optionally their content
• Run (subsets of) the tests.
  - Copy test document to separate directory.
  - Run multiple tests in parallel.
  - Compare with expectations
• Aggregate results and output a report (options for machine and human readability).

Bonus: Can we script/wrap the resulting test bank so that it can be automatically run by some CI tool? That would be helpful evaluate pull requests (if we should get any).