warning: the use of tmpnam is dangerous, better use mkstemp

[Saviq]

/usr/bin/ld: tests/miral/CMakeFiles/miral-test.dir/external_client.cpp.o: in function `(anonymous namespace)::ExternalClient::ExternalClient()':                                                                                              
/home/michal/dev/MirServer/mir/tests/miral/external_client.cpp:32: warning: the use of `tmpnam' is dangerous, better use `mkstemp'                                                                                                            
/usr/bin/ld: tests/miral/CMakeFiles/miral-test-internal.dir/static_display_config.cpp.o: in function `StaticDisplayConfig_nonexistent_config_file_is_no_error_Test::TestBody()':                                                              
/home/michal/dev/MirServer/mir/tests/miral/static_display_config.cpp:151: warning: the use of `tmpnam' is dangerous, better use `mkstemp' 

[AlanGriffiths]

I've no doubt that this can be done better, just think solving it is more than a "first issue": mkstemp() doesn't provide a filename that can be used with external_client.launch() or StaticDisplayConfig (and I've not found an incantation to disable the warning from the linker).