Disable tests using injection points in installcheck

Disable tests using injection points in installcheck

The 'gin' test injections faults to GIN index build. If another test
running concurrently in the same cluster also tries to create a GIN
index, it will hit the fault, too.

To fix, disable tests using injection points when running against an
existing cluster. A better long-term solution would be to make the
injection points scoped to the database or process, but this will do
for now.

Discussion: https://www.postgresql.org/message-id/CA%2BhUKGJYhcG_o2nwSK6r01eOZJwNWUJUbX%3D%3DAVnW84f-%2B8yamQ@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/10fd6cdd-c5d9-46fe-9fa1-7e661191309e@iki.fi