In the pg_upgrade test suite, don't write to src/test/regress.

When this suite runs installcheck, redirect file creations from
src/test/regress to src/bin/pg_upgrade/tmp_check/regress.  This closes a
race condition in "make -j check-world".  If the pg_upgrade suite wrote
to a given src/test/regress/results file in parallel with the regular
src/test/regress invocation writing it, a test failed spuriously.  Even
without parallelism, in "make -k check-world", the suite finishing
second overwrote the other's regression.diffs.  This revealed test
"largeobject" assuming @abs_builddir@ is getcwd(), so fix that, too.

Buildfarm client REL_10, released fifty-four days ago, supports saving
regression.diffs from its new location.  When an older client reports a
pg_upgradeCheck failure, it will no longer include regression.diffs.
Back-patch to 9.5, where pg_upgrade moved to src/bin.

Reviewed (in earlier versions) by Andrew Dunstan.

Discussion: https://postgr.es/m/20181224034411.GA3224776@rfd.leadboat.com