- Rename "getImportJob" to "createOrGetConfiguredImportJob" to better
express its behavior and also change its JavaDoc.
- Add a new method "getCurrentImportJob" that merely retrieves the
current job (without creating/configuring anything).
- Extract private method in SmartImportTests
- Remove calls to waitForJobs(...) in SmartImportTests and join the
SmartImportJob instead.

Fixes: https://github.com/eclipse-platform/eclipse.platform.ui/issues/1427
Fixes: https://github.com/eclipse-platform/eclipse.platform.ui/issues/1807