only enable RESTRICT fk constraint tests for supporting dialects

MSSQL doesn't support a RESTRICT fk constraint, so we need a way
to conditionally disable these tests. This change adds an extra
section to dialect.supports indicating support for RESTRICT
constraints. Also changed some cases where RESTRICT wasn't being
explicitly tested but used to test the ability to simply use
constraints (query-interface.test.js)
this is because the method is now "SET NULL" rather than "RESTRICT". The FK is being set to null, therefore the key length is one less than it used to be - a side effect of the change