The ExternalProject_Add(mavsdk_tests ...) lived at the bottom of
sitl_targets_gazebo-classic.cmake, outside the if(gazebo_FOUND) gate
and with a comment noting it doesn't depend on sitl_gazebo. Historical
artifact: mavsdk_tests is a black-box MAVLink test driver that talks
to a running PX4 over UDP, and works against any simulator backend
(sitl_gazebo-classic, gz, sih, jmavsim, ...).
With CI moving from classic Gazebo to SIH, the misplacement becomes
actively misleading. Pull the block into a new top-level
cmake/mavsdk_tests.cmake and include it from CMakeLists.txt under the
existing posix-only block, so the dependency graph reflects what is
actually true.
Behavior is unchanged: target is EXCLUDE_FROM_ALL, only built on
demand via `make mavsdk_tests` or `make tests_integration`.
