Update `TestcontainersLifecycleBeanPostProcessor` so that containers
can actually be started in parallel.
Prior to this commit, `initializeStartables` would collect beans
and in the process trigger the `postProcessAfterInitialization` method
on each bean. This would see that `startablesInitialized` was `true`
and call `startableBean.start` directly. The result of this was that
beans were actually started sequentially and when the `start` method
was finally called it had nothing to do.
The updated code uses an enum rather than a boolean so that the
`postProcessAfterInitialization` method no longer attempts to start
beans unless `initializeStartables` has finished.
Fixes gh-38831
6ae113c18a