Add unit test for dynamically loading modules on posix.

src/platforms/posix/tests/dyn_hello/CMakeLists.txt

@@ -0,0 +1,8 @@
+px4_add_module(
+	MODULE platforms__posix__tests__dyn_hello
+	MAIN hello
+	SRCS
+		hello.cpp
+	DEPENDS
+	DYNAMIC
+	)

src/platforms/posix/tests/dyn_hello/hello.cpp

@@ -0,0 +1,16 @@
+#include <px4_log.h>
+#include <px4_app.h>
+
+extern "C" __EXPORT int hello_main(int argc, char *argv[]);
+int hello_main(int argc, char *argv[])
+{
+	PX4_INFO("Hello, I am a dynamically loaded module.");
+
+	PX4_INFO("Argv:");
+
+	for (int i = 0; i < argc; ++i) {
+		PX4_INFO("  %d: %s", i, argv[i]);
+	}
+
+	return 0;
+}