Add some tests for the source view

The initial source view implementation landed without tests.

I promised Julien that I'd add at least a few tests as a follow-up. Here are two things that I'd like to cover:

* A basic test for the UI interaction: double-clicking in the call tree should open a bottom view and trigger a request for source code with fetch, and then display the code in the source view, with the correct lines of code in view.
* A test that makes sure that we use the call node's line timings for determining the hotspot scroll position. So I'll need to make an example profile which has different hotspots on different call nodes for the same function, and yet a different hotspot in the same file for some other function.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-488)