The UIEditWorkingSetWizardAuto#testEditPage randomly fails because the
project deletion during test cleanup fails. The project file of the
second project to be deleted is already locked.

To avoid that the deletion of the first project triggers some
asynchronous tasks that interfere with the deletion of the second
project, this change combines both deletions into an atomic workspace
operation. In addition, the change separates the exception handling for
both projects to see from the stack trace which of the deletions caused
the exception.

Contributes to
https://github.com/eclipse-platform/eclipse.platform.ui/issues/275