[Bug] Fix Backend UT Problem (#5784) (#5785)

1. relocation R_X86_64_32 against `__gxx_personality_v0' can not be used when making a shared object; recompile with -fPIC
2. warning: the use of `tmpnam' is dangerous, better use `mkstemp'
3. Death tests use fork(), which is unsafe particularly in a threaded context. For this test, Google Test couldn't detect the number of threads.

be/test/runtime/user_function_cache_test.cpp

@@ -96,7 +96,7 @@ public:
         hostname = "http://127.0.0.1:" + std::to_string(real_port);
 
         // compile code to so
-        system("g++ -shared ./be/test/runtime/test_data/user_function_cache/lib/my_add.cc -o "
+        system("g++ -shared -fPIC ./be/test/runtime/test_data/user_function_cache/lib/my_add.cc -o "
                "./be/test/runtime/test_data/user_function_cache/lib/my_add.so");
 
         my_add_md5sum =