Fix oscillations in Heun2 IMEX dense output

[wthrowe]

Also do a bunch of reorganization of the time-stepper test helpers.

Proposed changes

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[teukolsky]

In general, dense output only needs to be one order lower than the method itself, so from a theoretical point of view this linear interpolation should be fine.

[nilsdeppe]

If you want to do the clang-tidy suggestions please squash right away or if you don't please go ahead and merge this PR :)