Tutorial navigation selected state

[tcjr]

Fixes #1877

The Selection component shows the select/optgroup/options for all the tutorials. It gets the data to render the options from the grouped property of the DocsService and calls the transitionTo method on the RouterService when a tutorial is selected.

When rendering each option, the selected attribute was not taking into account the group, so it was comparing the tutorial path part with the full current path and they would never match.

This PR updates the isSelected helper so it accepts both the group and tutorial.

I also added some component tests that use mocks for the two injected services.

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[tcjr]

I couldn't figure out why assert.dom() was failing the TS lint.

Property 'dom' does not exist on type 'Assert'

[NullVoxPopuli]

do you have an example of how I can see the failure case in action?
I'm looking on the currently deployed site, and I'm not sure.

thank you for fixing this tho!
I just want to make sure I fully understand the problem

[tcjr]

The linked issue has a great video of the issue.

Essentially: select any tutorial using the selector. Then, open the selector again. Notice the checked tutorial did not update.

[NullVoxPopuli]

ah! my issue is that I have no checkmark on firefox 🙈 lol

alrighty -- I'll merge once green. thank you!