In order to avoid the repetitive creation of temporary workspace folders
in session tests, this change encapsulation the temporary folder
creation in a customization object for the session test extension. The
specification of a custom workspace folder is now optional. The
simplification is exemplarily applied to the WorkspacePreferencesTest.
The change also removes unnecessary static declarations of the
SessionTestExtension.

The design of the customization application already prepares for further
customization options, such as for customizing the workbench
configuration.

Contributes to
https://github.com/eclipse-platform/eclipse.platform/issues/903