video-* plugins: stim parameter name change and tests

[becky-gilbert]

These are the changes from @mgorenstein's pull request (#1171), merged into this branch so that I could also update the documentation and example HTML files with the new parameter name.

I also added tests to each of the plugin test files to make sure that (1) the plugin files are calling the registerPreload function, and (2) the parameter name that is used in that function exists in the plugin (to catch the stimulus/sources name error that was preventing video preloading before).

1. The easiest way I could think of to test whether an incorrect parameter name was used in jsPsych.pluginAPI.registerPreload was to modify jspsych.js so that it produces a console warning if the parameter in the preloads list is undefined, and then detect this console warning in the test. This test is working, i.e. it fails when jsPsych.pluginAPI.registerPreload uses a parameter name that doesn't exist in the trial object and passes when stimulus is used as the parameter name.

2. I couldn't get the jest.spyOn() function to work for spying on jsPsych.pluginAPI.registerPreload. I thought that this would work:const preload_spy = jest.spyOn(jsPsych.pluginAPI, 'registerPreload'); //... set up and run video trial expect(preload_spy).toHaveBeenCalled(); But this always fails, saying that preload_spy was not called. I kept this in the code but commented out the expect line to make the tests pass. @jodeleeuw any ideas/suggetions?

Also @jodeleeuw I realize that this sort of testing isn't quite what you meant in this comment, but figured this was the quickest way to see if I can get the preload tests working. I can test this in a different way and move the tests somewhere else if you prefer. I think including preload tests somewhere will be useful for #1258.

[jodeleeuw]

I wonder if the reason that .spyOn isn't working is because the plugin file is loading before the spy is created. The registerPreload method is called when the plugin is loaded for the first time. I believe in most of the tests we load the plugin file before running the test. Maybe for this test we need to load jsPsych, set up the spy, and then load the plugin.

[becky-gilbert]

@jodeleeuw yep that was it - fixed the tests now. Thanks!
Is this ok to be merged?