test: cleanup test-fs-watch.js

Reversed "actual" and "expected" arguments for assert.strictEqual().

Replaced constructor with regular expression for assert.throws().

PR-URL: https://github.com/nodejs/node/pull/12595


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>