tests/frontend: Re-introduce tests for `track --except`

Previously, the --except functionality was only tested as part of build --track. And, accidentally got removed while support for --track was being ripped out from bst --build.

Add a new and improved version of that test, using source track command instead, and also exercising the --deps option in conjunction with --except. In future, if we support more values for --deps (like build and run), it should be relatively easy to extend the same test.

This was originally brought up in !1893 (comment 335696186).

Also, includes a minor refactor for the generate_element in the same module. I have currently added that as a separate commit for ease of review. Happy to squash it if people would prefer to have the whole thing as a single commit.