create tag for hets-testing

The hets-testing tag sets Sidekiq test mode to inline (immediate
execution). It shall also allow us to exclude rather slow hets tests on
a test-run.

'Default' Sidekiq::Testing.inline! was removed, because
it didn't work anyway (because of scoping).
process async jobs in spec inline by using tags

The needs_hets and process_async tags are used
to process asynchronous jobs inline.
Also remove all unnecessary Sidekiq::Testing.fake!
blocks.
semantically annotate hetsspec with needs_hets-tag.