Take Approval test in use for SeleniumLibrary unit tests.

[aaltat]

ApprovalTests is an interesting concept, which would help writing unit tests. Unfortunately, adding ApprovalTests as a dev dependency does not work out of the box or like #1147 . This is because ApprovalTests does not work with Jython and there is bug with pypy compatibility. But the issue with pypy should be resolved if my PR is approved.

To take approval test in use.

1. Update unit test documentation
2. Solve the problem with Jython
3. Wait for pypy support release
4. Write some example tests.

The Jython problem is difficult to solve, because there is import error during the installation: ImportError: cannot import name c_wchar_p. But it could be solved in following way

1. Tune Jython installation in Travis not to install ApprovalTests
2. In unit test, make try/except in import of ApprovalTests
3. Create skip for unit test if running with Jython and use it when running test with ApprovalTests.

[aaltat]

Done in #1173