Customise launchd plist location on macOS

Currently, the plist location appears to be hard-coded in

https://github.com/facebook/watchman/blob/34ac399bbcc8841692e9d7d7c8807efc772a5100/watchman/main.cpp#L431-L440

Is it possible to allow this location to be specified, perhaps as a command-line option?

One use-case for this is to allow proper testing in Homebrew. Currently, our test is limited to checking the output of `watchman -v`, because our testing sandbox limits the locations you can write to, and this does not include `~/Library/LaunchAgents`. However, since we build watchman against Homebrew-provided dependencies it would be good to have a proper test of functionality here.