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

test/CMakeLists.txt

@@ -0,0 +1,23 @@
+px4_add_git_submodule(TARGET git_fuzztest PATH "fuzztest")
+
+message(STATUS "Adding fuzztest")
+# This will also add GTest
+add_subdirectory(fuzztest EXCLUDE_FROM_ALL)
+
+# Ensure there's no -R without any filter expression since that trips newer ctest versions
+if(TESTFILTER)
+    set(TESTFILTERARG "-R")
+else()
+    set(TESTFILTERARG "")
+endif()
+
+add_custom_target(test_results
+        # antlr4_tests_NOT_BUILT gets added by fuzztest
+        COMMAND GTEST_COLOR=1 ${CMAKE_CTEST_COMMAND} --output-on-failure -T Test ${TESTFILTERARG} ${TESTFILTER} --exclude-regex "antlr4_tests_NOT_BUILT"
+        DEPENDS
+        px4
+        examples__dyn_hello
+        USES_TERMINAL
+        COMMENT "Running tests"
+        WORKING_DIRECTORY ${PX4_BINARY_DIR})
+set_target_properties(test_results PROPERTIES EXCLUDE_FROM_ALL TRUE)