build: disable fuzztest when building with TSAN

fuzztest's coverage instrumentation is incompatible with Thread
Sanitizer. Add px4_setup_gtest_without_fuzztest() macro to
cmake/px4_add_gtest.cmake that fetches GTest standalone and stubs out
fuzztest cmake functions. Guard all fuzztest-specific code on
TARGET fuzztest::fuzztest so it compiles cleanly without fuzztest.

platforms/posix/CMakeLists.txt

@@ -28,7 +28,7 @@ add_executable(px4
 	apps.cpp
 	)
 
-if (BUILD_TESTING)
+if (BUILD_TESTING AND TARGET fuzztest::fuzztest)
 	# Build mavlink fuzz tests. These run other modules and thus cannot be a functional/unit test
 	add_executable(mavlink_fuzz_tests EXCLUDE_FROM_ALL
 		src/px4/common/mavlink_fuzz_tests.cpp

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

@@ -36,4 +36,9 @@ set(SRCS
    )
 
 px4_add_library(gtest_functional_main ${SRCS})
-target_link_libraries(gtest_functional_main PUBLIC gtest fuzztest::init_fuzztest)
+if(TARGET fuzztest::init_fuzztest)
+	target_link_libraries(gtest_functional_main PUBLIC gtest fuzztest::init_fuzztest)
+	target_compile_definitions(gtest_functional_main PRIVATE HAVE_FUZZTEST)
+else()
+	target_link_libraries(gtest_functional_main PUBLIC gtest)
+endif()

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

@@ -32,7 +32,9 @@
  ****************************************************************************/
 
 #include <gtest/gtest.h>
+#ifdef HAVE_FUZZTEST
 #include <fuzztest/init_fuzztest.h>
+#endif
 
 #include <uORB/Subscription.hpp>
 
@@ -42,8 +44,10 @@ int main(int argc, char **argv)
 {
 	testing::InitGoogleTest(&argc, argv);
 
+#ifdef HAVE_FUZZTEST
 	fuzztest::ParseAbslFlags(argc, argv);
 	fuzztest::InitFuzzTest(&argc, &argv);
+#endif
 
 	uORB::Manager::initialize();
 	param_init();