Add unit tests for instructor tool

[itsjeyd]

This PR aims to extend unit test coverage for problem-builder by introducing unit tests for the Instructor Tool block.

It follows up on #47.

[itsjeyd]

@e-kolpakov Could you please have a look at what I have so far? With the unit test introduced by 1f603db I'm verifying that individual questions are represented correctly in the root block dropdown. The basic idea is to check whether the Fragment returned by student_view contains a line like this for each question that it's supposed to display:

<option value="block_id" ...>... block_name ...</option>

... where block_name corresponds to the value of the block attribute that is preferred for display purposes. (For example, if the value of block.question is non-empty, we always want to display that instead of block.name, block.display_name, or block_id).

Maybe there's a simpler approach? I haven't worked with the mock library a lot, so I'm probably not using it to its full potential.

CC @antoviaque

[e-kolpakov]

@itsjeyd I believe the test tests too much:

• student_view method builds correct render context
• template engine uses the context to render expected select options.

I can see two ways to improve (and potentially simplify) it:

1. Convert it to integration test, than use selenium stuff to check for option values/texts
2. Patch loader.render_template and make sure it get's the correct rendering context.

Since we have an idea to have less integration and more unit tests I would go for option 2. It lacks testing that template does what it supposed to do, but I don't know any way to test templates except integration tests (if you happen to know one - share the knowledge :))

[itsjeyd]

@e-kolpakov Thanks for the feedback! I went for option 2. If you could have a quick look to see if the changes match what you had in mind, that would be great :)

[itsjeyd]

@e-kolpakov I added six more unit tests and two integration tests addressing issues that came up for #47. Please review. :)

[e-kolpakov]

@itsjeyd nit: could use a helper method:

def check_block(usage_id, expected_name):
    matching_blocks = [block for block in block_tree if block['id'] == self.block.scope_ids.usage_id]
    self.assertEqual(len(matching_blocks), 1)
    matching_block = matching_blocks[0]
    self.assertEqual(matching_block['name'], expected_name)

[itsjeyd]

@e-kolpakov Thanks for the comments! I addressed them all. Please have another look at the code.

[e-kolpakov]

@itsjeyd 👍