PHPLIB-1750 Relax search index management spec test to allow the extra 'field' property

[GromNaN]

Fix PHPLIB-1750

The docker image mongodb/mongodb-atlas-local have a different behavior than MongoDB Atlas.

This change makes the test less sensible to the server output.

Fixed test:

Expected :'{ "mappings" : { "dynamic" : false } }'
Actual   :'{ "mappings" : { "dynamic" : false, "fields" : { } } }'