Additional fix for OPENDJ-1361: Matching Rules and Syntaxes have a reference to a schema that can be changed, leading to unexpected behavior

* SchemaBuilder: use Reject for ensuring that matching rules and syntaxes have not already been validated when they are added to a schema builder
* Syntax: inherit matching rules from default syntax
* SyntaxTestCase: remove invalid testCreatesEmptySyntax() test, add two tests for default syntax substitution.