test: add google fuzztest to unit test build

From https://github.com/google/fuzztest.
This will now also add gtest (via cmake FetchContent)

And requires newer cmake (container updates):
CMake 3.19 or higher is required.  You are running version 3.16.3

platforms/posix/src/px4/common/gtest_runner/CMakeLists.txt

@@ -36,4 +36,4 @@ set(SRCS
    )
 
 px4_add_library(gtest_functional_main ${SRCS})
-target_link_libraries(gtest_functional_main PUBLIC gtest)
+target_link_libraries(gtest_functional_main PUBLIC gtest fuzztest::init_fuzztest)

platforms/posix/src/px4/common/gtest_runner/gtest_functional_main.cpp

@@ -32,6 +32,7 @@
  ****************************************************************************/
 
 #include <gtest/gtest.h>
+#include <fuzztest/init_fuzztest.h>
 
 #include <uORB/Subscription.hpp>
 
@@ -41,6 +42,9 @@ int main(int argc, char **argv)
 {
 	testing::InitGoogleTest(&argc, argv);
 
+	fuzztest::ParseAbslFlags(argc, argv);
+	fuzztest::InitFuzzTest(&argc, &argv);
+
 	uORB::Manager::initialize();
 	param_init();
 	return RUN_ALL_TESTS();