Fix system tests

What does this MR do?

The system tests were not active in the CI since the last major CMake refactor !49 (merged).

• This was an oversight. The CMake conditional shall check for TARGET. However, even if this is fixed, dune-testtools did not allow to make tests over aliased targets because it wanted to modify the EXCLUDE_FROM_ALL property. This was only solved recently in quality/dune-testtools!141 (merged)

Can this MR be accepted?

• I've read contribution guide
• Implemented
  • Make exec targets available on main directory
  • Make alias targets available on test directory
  • quality/dune-testtools!141 (merged) merged into releases/2.7
• Added/Updated tests
  • Fixed new NewtonMethod exceptions
• Added/Updated documentation
• Pipelines passing
• Delete branch option set
• Fast-forwaded git merge is possible
• Added entry to CHANGELOG.md

Related issues

Closes #