ReadConcern tests need not exhaustively test each constant

The ReadConcern tests (e.g. debug output, serialization) currently test all of the constants, even though ReadConcern itself is not limited to using those constants. Its constructor accepts any string as a level. These tests should be stripped down to just test a single level string. Only readconcern-constants.phpt need exhaustively test each constant value.

This came up in mongodb/mongo-php-driver#1243 for PHPC-1761 when a new constant was being added.