move mixer_test and conversion_test to gtest

unittests/conversion_test.cpp

@@ -0,0 +1,9 @@
+#include <systemlib/mixer/mixer.h>
+#include <systemlib/err.h>
+#include "../../src/systemcmds/tests/tests.h"
+
+#include "gtest/gtest.h"
+
+TEST(ConversionTest, FMU_quad_w) {
+        ASSERT_EQ(test_conv(0, NULL), 0) << "Conversion test failed";
+}