Rewrite WDF test TestSequence.testAllocSize

The test
org.eclipse.persistence.testing.tests.wdf.jpa1.generator.TestSequence.testAllocSize
made incorrect assumptions on implementation details and therefore
failed. This change changes the test implementation.

The new test is no longer skipped on Oracle and MaxDB platforms.
However, it is skipped when executed inside an application server.

Signed-off-by: Sabine Heider <sabine.heider@sap.com>