Fix tests and re-introduce tests into build

cmake/configs/posix_sitl_default.cmake

src/systemcmds/tests/test_adc.c

src/systemcmds/tests/test_bson.c

src/systemcmds/tests/test_conv.cpp

src/systemcmds/tests/test_file.c

src/systemcmds/tests/test_file2.c

src/systemcmds/tests/test_gpio.c

src/systemcmds/tests/test_hott_telemetry.c

src/systemcmds/tests/test_hrt.c

src/systemcmds/tests/test_jig_voltages.c

src/systemcmds/tests/test_led.c

src/systemcmds/tests/test_mathlib.cpp

src/systemcmds/tests/test_mixer.cpp

src/systemcmds/tests/test_mount.c

src/systemcmds/tests/test_params.c

src/systemcmds/tests/test_perf.c

src/systemcmds/tests/test_ppm_loopback.c

src/systemcmds/tests/test_rc.c

src/systemcmds/tests/test_sensors.c

src/systemcmds/tests/test_servo.c