Windows watch mode: fix a small race condition, re-enable tests

[nojb]

This PR contains a small fix for the Windows watch mode #7010 that can result in some notifications being lost when they arrive immediately after the watched directory is added. It also re-enables a unit test for this watcher backend.

[rgrinberg]

@MisterDA could you take a look?

[nojb]

Description of the changes for the prospective reviewer:

1. when adding a new directory, the OCaml thread sends a message to the native thread to do the actual addition. This means that there is a delay between calling Fswatch_win.add and the actual start of the monitoring. This can cause some events to be missed if the events arrive very quickly after calling Fswatch_win.add (this was the case in the unit test). To fix this, we make sure that Fswatch_win.add does not return until monitoring is actually started in the other thread. We communicate this fact using an event object (afterAdd).
2. For the same reason, it is not a good idea to pass relative paths to Fswatch_win.add (as the current directory could change between the call to Fswatch_win.add and the actual point in time when monitoring starts). Of course with 1) this should not happen, but for extra safety I enforce that directories passed to Fswatch_win.add be absolute (this is the case in current Dune codebase).

[jonahbeckford]

Just saw this. Will have a review tomorrow.

[jonahbeckford]

Looks great! Thanks.

[nojb]

@emillon this one can go in 3.7.1.

[emillon]

done in #7474