injection_points: Add basic isolation test

injection_points: Add basic isolation test

This test can act as a template when implementing an isolation test with
injection points, and tracks in a much simpler way some of the behaviors
implied in the existing isolation test "inplace" that has been added in
c35f419d6efb.  Particularly, a detach does not affect a backend wait; a
wait needs to be interrupted by a wakeup.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/ZxGTONm_ctQz--io@paquier.xyz