Replace long-running tests with small regression tests

[brechtm]

This requires upgrading to a newer Sphinx version (submodule).

[brechtm]

I've come to the conclusion that the long-running tests aren't really practical. They take a long time to execute and the documents are so long that they often fail to produce a useful PDF-diff. It's better to increase the scope of the small regression tests.

These elements present in demo.txt are missing from the small regression tests:

• [ ] transition
• [ ] definition list
• [ ] field list
• [ ] option list
• [ ] quoted literal block
• [ ] line block
• [ ] block quote
• [ ] doctest block
• [x] table (with and without caption)
• [ ] target
• [ ] inline table of contents (document parts)
• [ ] figure
• [ ] topic
• [ ] sidebar
• [ ] rubric
• [ ] compound paragraph
• [ ] error
• [ ] inline
  • [ ] replacement text
  • [ ] citation
  • [ ] inline target