PHPLIB-1404: Convert retryable reads spec tests to unified test format (#1247)

Synced with mongodb/specifications@85fa23f

* Remove legacy retryable reads spec tests

* Allow document type for mapReduce out option

* Eagerly iterate MapReduceResult when executing operation

* Allow empty entity reference in preventStaleDbVersionError()

This works around an error for a distinct operation that references an entity created by a createEntities test runner operation.

Author: jmikola