PHP-819: Fix MongoCollection::setWriteConcern() and split tests

Fixes a bug from cd67c8910b483b6ef5324589b5e0c33b6e4325fb, where integer $w arguments to setWriteConcern() were set as 3.

This also splits the MongoDB and MongoCollection tests, and adds additional test cases for string $w values, inheriting MongoClient (parent) write concerns, and setting the timeout.
Merge pull request #609