Re-enable SchemaValidationTest for Maria and MySQL

[DavideD]

The test is currently disabled:

hibernate-reactive/hibernate-reactive-core/src/test/java/org/hibernate/reactive/schema/SchemaValidationTest.java

Line 54 in 69fa4fc

@DisabledFor(value = { MARIA, MYSQL }, reason = "HHH-18869: Schema creation creates an invalid schema")

This was due to an issue in ORM that seems to be solved now: https://hibernate.atlassian.net/browse/HHH-18869