Move unittests into a more perceivable directory

unittests/mixer_test.cpp

@@ -0,0 +1,14 @@
+#include <systemlib/mixer/mixer.h>
+#include <systemlib/err.h>
+#include "../../src/systemcmds/tests/tests.h"
+
+int main(int argc, char *argv[]) {
+	warnx("Host execution started");
+
+	char* args[] = {argv[0], "../../ROMFS/px4fmu_common/mixers/IO_pass.mix",
+				 "../../ROMFS/px4fmu_common/mixers/FMU_quad_w.mix"};
+
+	test_mixer(3, args);
+
+	test_conv(1, args);
+}