tests: add a wrapper to run fsmonitor tests This script does a bunch of non-trivial configuration work: in particular, it sets up an isolated instance of Watchman which isn't affected by global state and can be torn down on completion. This script also sets the HGFSMONITOR_TESTS environment variable, which hghave will use in the next patch to allow gating on whether fsmonitor is enabled. With fsmonitor enabled, there appear to be a number of failures in the test suite. It's not yet clear to me why they're happening, but if someone would like to jump in and fix some of them I hope this will be helpful for that.