CM_MongoDb_ClientTest::testDatabaseExists is flaky

As reported by @mariansollmann 

I can reliably reproduce it by running all tests, then only the test in question:
```
bin/phpunit
bin/phpunit --filter 'CM_Mongo_ClientTest::testDatabaseExists'
```

@mariansollmann @alexispeter @kris-lab  any takers?

Tipp: maybe we can identify which test is causing the side effect by only running half of the tests in the first step, with:
```
bin/phpunit --filter "/^CM_(Action|AdproviderAdapter|App|Asset|Cache|Class|Cli|Clockwork|Config|Db|Dom|Elasticsearch|EventHandler|ExceptionHandling|File|Form|FormAction|FormField|Frontend|Geo|InputStream|Jobdistribution|Layout|Mail|Model|ModelAsset|MongoDb)/"
```
Then breaking that again in half, etc..