Remove unneeded newline append in emrun when logging to file.
Fix potential race condition in emrun where the browser was spawned before the web server, and theoretically browser could attempt to load the page before server is even up. Not detected in practice, but could happen.
Improve emrun browser launch detach detection on linux. Shield log printing with mutex to not interleave multiple prints to same line without a delimiting newline.
In browser.test_emrun, use --log_stdout and --log_stderr to capture run output instead of python pipe, since on linux, spawning chrome browser would run into a mysterious issue where piping the emrun output would hang the python subprocess spawn.