Improve testing framework

[leoisl]

This is the test coverage report from PR #1, which adds more tests to BIGSI, sorted by source file with lowest test coverage to highest:
Coverage report.pdf
We should improve the test coverage in many files.

I wonder if you think that the tests are cumbersome to run, due to using real DBs. One option would be to mock the DBs, but it is nice to have tests using real DBs also. I see that many of the tests are actually integration tests, as they test not only the function/method being tested, but all the other functions and methods called by the tested function/method. I could start adding unit testing with mocking if you wish, although it seems that integration tests might be more complete, but less specific, than unit testing.

Anyway, this issue is more like a discussion issue to talk about how we should improve the testing framework.