Force run of pg_upgrade in the build directory in its TAP test

TAP tests are run from their own directory in the source tree, and in a
VPATH build the execution of the pg_upgrade command was leaving behind a
file in the source tree, that should be left untouched.  In order to
avoid this issue, the test moves to PostgreSQL::Test::Utils::tmp_check,
so as any files generated by pg_upgrade do not impact the source tree,
but the build tree.  This has as nice side-effect to make unnessary the
presence of such files in pg_upgrade's .gitignore and Makefile.  This
strategy is similar to psql's test 010_tab_completion.pl, though the
reasons behind this choice are different.

In passing, fix one misleading test name that was added by 99f6f19.

Per discussion with Peter Eisentraut, Andrew Dunstan, Tom Lane, Andres
Freund and myself.

Discussion: https://postgr.es/m/f80ace33-11fb-1cd3-20f8-98f51d151088@enterprisedb.com

src/bin/pg_upgrade/.gitignore

@@ -1,7 +1,4 @@
 /pg_upgrade
 # Generated by test suite
-/delete_old_cluster.sh
-/delete_old_cluster.bat
-/reindex_hash.sql
 /log/
 /tmp_check/

src/bin/pg_upgrade/Makefile

@@ -48,8 +48,7 @@ uninstall:
 
 clean distclean maintainer-clean:
 	rm -f pg_upgrade$(X) $(OBJS)
-	rm -rf delete_old_cluster.sh log/ tmp_check/ \
-	       reindex_hash.sql
+	rm -rf log/ tmp_check/
 
 check:
 	$(prove_check)

src/bin/pg_upgrade/t/002_pg_upgrade.pl

@@ -206,6 +206,11 @@ if (defined($ENV{oldinstall}))
 	}
 }
 
+# In a VPATH build, we'll be started in the source directory, but we want
+# to run pg_upgrade in the build directory so that any files generated finish
+# in it, like delete_old_cluster.{sh,bat}.
+chdir ${PostgreSQL::Test::Utils::tmp_check};
+
 # Upgrade the instance.
 $oldnode->stop;
 command_ok(
@@ -238,7 +243,7 @@ $newnode->command_ok(
 		'-d',         $newnode->connstr('postgres'),
 		'-f',         "$tempdir/dump2.sql"
 	],
-	'dump before running pg_upgrade');
+	'dump after running pg_upgrade');
 
 # Compare the two dumps, there should be no differences.
 my $compare_res = compare("$tempdir/dump1.sql", "$tempdir/dump2.sql");