And add tests for verifying which files are and aren't reacted to,
including coverage for the existing ignoring of events from
"node_modules".

This is prep for #1669.

…ories

Previously the callback would return true for "cwd/node_modules",
then return false for all its immediate sub directories, and also
for all its indirect subdirectories. While these directories were
eventually watched, it could sometimes take considerable time to
iterate over these.

Use node-watch's skip feature to improve on this. And while at it,
move it up a level by ignoring the top-level node_modules as well,
not merely paths that we happen to see somewhere within it. Do both
just in case we don't see the top level for some reason.

Change `run.watch()` to also apply the `--require` file, so that we
can observe any changes to `require.extensions`.

Fixes #1669.