fix(mavlink): remove all stale mavlink_tests references

The mavlink_tests module was deleted in 1009268d31 but several
references were left behind, breaking builds on all targets.

Removed:
- CMakeLists.txt: add_subdirectory(mavlink_tests)
- mavlink_ftp.cpp: #include of deleted mavlink_ftp_test.h
- mavlink_ftp.h: MavlinkFtpTest forward decl and friend class
- posix-configs/SITL/init/test/test_mavlink: dead init script
- sitl_tests.cmake: sitl-mavlink CTest target
- install-voxl.sh: px4-mavlink_tests symlink

Ref: https://github.com/PX4/PX4-Autopilot/issues/26738
Signed-off-by: Ramon Roche <mrpollo@gmail.com>

src/modules/mavlink/mavlink_ftp.cpp

@@ -43,8 +43,6 @@
 #include <cstring>
 
 #include "mavlink_ftp.h"
-#include "mavlink_tests/mavlink_ftp_test.h"
-
 #include "mavlink_main.h"
 
 using namespace time_literals;