Fix bit-rot in pg_upgrade's test.sh, and improve documentation.

Fix bit-rot in pg_upgrade's test.sh, and improve documentation.

Doing a cross-version upgrade test with test.sh evidently hasn't been
tested since circa 9.2, because the script lacked case branches for
old-version servers newer than 9.1.  Future-proof that a bit, and
clean up breakage induced by our recent drop of V0 function call
protocol (namely that oldstyle_length() isn't in the regression
suite anymore).

(This isn't enough to make the test work perfectly cleanly across
versions, but at least it finishes and provides dump files that
you can diff manually.  One issue I didn't touch is that we might
want to execute the "reindex_hash.sql" file in the new DB before
dumping it, so that the hash indexes don't vanish from the dump.)

Improve the TESTING doc file: put the tl;dr version at the top not
the bottom, and bring its explanation of how to run a cross-version
test up to speed, since the installcheck target isn't there and won't
be resurrected.  Improve the comment in the Makefile about why not.

In passing, teach .gitignore and "make clean" about a couple more
junk output files.

Discussion: https://postgr.es/m/14058.1496892482@sss.pgh.pa.us