Add tests for event handlers

[GeorgMeyer23]

• Test event 'click' and check if method 'openDialogue' is executed.
• Test event 'change' and check if method 'displayFileName' is executed.
• Test event 'change' and check if method 'displayFileName' updates DOM.

[GeorgMeyer23]

@ain failed test is commented out (see test.js line 34, 37-40) Fails with '0 is not equal 1'.