Conditionally disable unit tests for PostgreSQL and MySQL

junit introduced Assume functionality to conditionaly skipping tests execution.
Use it to conditionaly skip tests for databases that may not be installed or
configured on current machine:

Running com.google.gwtorm.schema.sql.DialectPostgreSQLTest
Tests run: 4, Failures: 0, Errors: 0, Skipped: 4, Time elapsed: 0.015 sec

"Skipped: 4" indicates here that the tests were skipped, because of Assume
assumption was not satisfied.

Change-Id: If4ae2aaec944bc4035906dbdf677da84d734e1c6
2 files changed