Previously, when an embedded test database was being auto-configured
any schema username and password specified by the user would result
in a separate embedded test database being created to load the schema.
This then left the actual test database without the schema causing
test failures.

This commit updates the test database auto-configuration to set the
schema username to an empty string in a property source that's added
first to the environment's property sources. This causes any schema
username configured by the user to be ignored, preventing the creation
of a separate database for schema.sql processing.

Fixes gh-19321