Adapt integration tests and CI to HyTeG's new CMake configuration

• Includes HyTeG as a system dependency to suppress warnings triggered from within HyTeG.
• Removes waLBerla_import() as it is no longer needed.
• Enables host optimizations directly: Since hyteg!875 (merged), the correct way to enable host optimizations is to pass the compiler flag directly to CMake, by e.g. -DCMAKE_CXX_FLAGS=-march=native.

See discussion at hyteg#291 (closed).

added CI label

requested review from @thoennes

assigned to @he66coqe

approved this merge request

merged

mentioned in commit 1fc4cda1