Adapt for gaudi/Gaudi!1632

gaudi/Gaudi!1632 (merged) removes the support for legacy GaudiExamples test helpers (renamed as GaudiTestSuite).

This MR adapts to the new names (works with v39 too).

I also added dependency declarations for pytest, needed when building in a super-project context.

See also LHCb!5179 (merged)