When initializing runtime files (syntax files etc) in tests, initialize built-in runtime files only, to ensure that the tests are not affected by whatever is in ~/.config/micro/ on the test machine. micro_test.go already ensures that, by using its own temporary directory as an (empty) config directory. So we only need to fix buffer_test.go and rtfiles_test.go. In those tests, don't repeat the same dance with a temporary directory, instead just ignore the config directory.