Try to unbreak injection-fault tests in the buildfarm

Try to unbreak injection-fault tests in the buildfarm

The buildfarm script attempts to run all tests marked as
NO_INSTALLCHECK under src/test/modules without paying attention to
whether they are enabled or disabled in the parent Makefile. That
hasn't been a problem so far, because all the tests marked with
NO_INSTALLCHECK ran unconditionally in "make check". But commit
e2e3b8ae9e changed that: the injection fault tests are marked as
NO_INSTALLCHECK, and also depend on --enable-injection-points.

Try to work around that by ensuring that "make check" does nothing in
the those subdirectories. We can hopefully get rid of this hack soon,
after fixing the buildfarm client, or by switching to meson.

Discussion: https://www.postgresql.org/message-id/8e4cf596-dd70-432e-9068-16466ed596ed%40iki.fi