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 forty-five 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 by Andrew Dunstan.

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

src/test/regress/input/largeobject.source

@@ -203,12 +203,12 @@ END;
 
 SELECT lo_export(loid, '@abs_builddir@/results/lotest.txt') FROM lotest_stash_values;
 
-\lo_import 'results/lotest.txt'
+\lo_import '@abs_builddir@/results/lotest.txt'
 
 \set newloid :LASTOID
 
 -- just make sure \lo_export does not barf
-\lo_export :newloid 'results/lotest2.txt'
+\lo_export :newloid '@abs_builddir@/results/lotest2.txt'
 
 -- This is a hack to test that export/import are reversible
 -- This uses knowledge about the inner workings of large object mechanism
@@ -223,7 +223,7 @@ TRUNCATE lotest_stash_values;
 
 \lo_unlink :newloid
 
-\lo_import 'results/lotest.txt'
+\lo_import '@abs_builddir@/results/lotest.txt'
 
 \set newloid_1 :LASTOID