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>

posix-configs/SITL/init/test/test_mavlink

@@ -1,23 +0,0 @@
-#!/bin/sh
-# PX4 commands need the 'px4-' prefix in bash.
-# (px4-alias.sh is expected to be in the PATH)
-. px4-alias.sh
-
-param load
-param set CBRK_SUPPLY_CHK 894281
-
-dataman start
-
-battery_simulator start
-simulator_mavlink start
-tone_alarm start
-pwm_out_sim start
-
-ver all
-
-mavlink start -x -u 14556 -r 2000000
-mavlink boot_complete
-
-mavlink_tests
-
-shutdown