tests: fix a test hang on Windows when setting a debuglock I have no idea why, but running the `hg -R auto-upgrade debuglock --set-lock` command near the end of `test-upgrade-repo.t` hangs the test. It does background the process and `killdaemons.py` runs without error, but control doesn't return to `run-tests.py` until the process is manually killed. I did notice that `$!` in MSYS is *not* the PID of the process that got backgrounded, even when a simple `sleep 60 &` is run in MSYS without the *.t file. When `killdaemons.py` is run manually with the PID in ProcessExplorer, the backgrounded process terminates immediately, and returns control to `run-tests.py`. This looks like it would be a race, but the test waits 10s for the lock file to appear before attempting to kill the process, so there's time. `hg serve` has a `--pid-file` option to write the pid to the file, but this is only a debug command, so I'm not bothering with cluttering the command line.