Moved the existing select tests to a dedicated testcase
Added a test covering the selection with reset in multiple select boxes
Added tests ensuring setValue can be used for select boxes
Moved the test for select options without values to SelectTest

Having this test in the JS tests does not make sense. The test does not
involve any javascript.
Added the verification of isSelected for options without values
Added the description of the behavior of getValue on option elements

NodeElement::selectOption is expecting this behavior, which is indeed
necessary as the value of an option does not only depend on the value
attribute.
Added some safeguard for not found elements in the select tests
Merge pull request #555 from stof/select_tests

Tests for select boxes