testserver: cmd with `--log` option does not generate `listen-url` in tests

After replacing `--logtostderr` to `--log="{file-defaults: {dir: tmp/dir/to/log}, sinks: {stderr: {filter: NONE}}}"` , the "cockroach" command does not generate the `listen-url` file in the tmp directory when running tests from `testserver_test.go`. But the same command works well in local CLI.

Demo of tests with original `--logtostderr` running in debugger: check [here](https://www.dropbox.com/s/jzg199pdl0c75fi/cmd_with_logerr.mp4?dl=0)
Demo of tests with `--log` running in debugger and local CLI: check [here](https://www.dropbox.com/s/1jf5aj4kzxbaovs/log_flag_in_cmd.mp4?dl=0)

Demos are based on[ these code changes](https://github.com/ZhouXing19/cockroach-go/commit/8e13e65b61f3d69a60f8eea975530b84584c4b6d).