test: improve test-fs-open-flags

* use arrow funcion instead of function expression
* add second argument to method assert.throws
* check error messages from beginning to the end using ^ and $

PR-URL: nodejs#10908
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Adrian Estrada <edsadr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>