tests: separate out code that initializes watchman files into a mixin
tests: separate out code that initializes watchman files into a mixin

Setting up the files beforehand is generally useful, but unfortunately it
doesn't catch issues with state directory creation or permissions. So we need
to have a separate kind of instance, meant specifically to test state dirs.
Separating this code out into a mixin will allow us to also plug in a separate
mixin later.

nodakai/watchman,dhruvsinghal/watchman,wez/watchman,nodakai/watchman,kwlzn/watchman,nodakai/watchman,dhruvsinghal/watchman,dhruvsinghal/watchman,kwlzn/watchman,nodakai/watchman,wez/watchman,wez/watchman,dhruvsinghal/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,facebook/watchman,dhruvsinghal/watchman,facebook/watchman,facebook/watchman,facebook/watchman,wez/watchman,facebook/watchman,wez/watchman,facebook/watchman,wez/watchman,kwlzn/watchman,kwlzn/watchman,kwlzn/watchman,wez/watchman,kwlzn/watchman,dhruvsinghal/watchman,dhruvsinghal/watchman,nodakai/watchman,facebook/watchman,kwlzn/watchman,dhruvsinghal/watchman