Regression test coverage of GiST index build is awful

This tells a pretty scary story:

https://coverage.postgresql.org/src/backend/access/gist/index.html

In particular, gistbuildbuffers.c is not entered *at all*, and
gistbuild.c is only 21% covered.

I noticed this after adding an assertion that I expected
gistInitBuildBuffers to fail on, and nonetheless getting
through check-world just fine.

Why is this so bad?  It's not like the gist regression test isn't
ridiculously expensive already; I'd have expected it to provide
darn near 100% coverage for what it's costing in runtime.

            regards, tom lane