Clean up pg_rewind regression test script.

Since commit 01f6bb4b2, TestLib.pm has exported path to tmp_check directory,
so let's use that also for the pg_rewind test clusters etc.

Also, in master, the $tempdir_short variable has not been used since commit
13d856e17, which moved the initdb-running code to TestLib.pm.

Backpatch to 9.5.

src/bin/pg_rewind/RewindTest.pm

@@ -62,15 +62,8 @@ our @EXPORT = qw(
   clean_rewind_test
 );
 
-# A temporary directory created with 'tempdir' is deleted automatically at
-# the end of the tests. You can change it to a constant if you need to keep it
-# for debugging purposes,
-my $testroot = tempdir;
-
-our $test_master_datadir  = "$testroot/data_master";
-our $test_standby_datadir = "$testroot/data_standby";
-
-mkdir $testroot;
+our $test_master_datadir  = "$tmp_check/data_master";
+our $test_standby_datadir = "$tmp_check/data_standby";
 
 # Define non-conflicting ports for both nodes.
 my $port_master  = $ENV{PGPORT};
@@ -273,9 +266,8 @@ sub run_pg_rewind
 
 	# Keep a temporary postgresql.conf for master node or it would be
 	# overwritten during the rewind.
-	copy(
-		"$test_master_datadir/postgresql.conf",
-		"$testroot/master-postgresql.conf.tmp");
+	copy("$test_master_datadir/postgresql.conf",
+		 "$tmp_check/master-postgresql.conf.tmp");
 
 	# Now run pg_rewind
 	if ($test_mode eq "local")
@@ -307,9 +299,8 @@ sub run_pg_rewind
 	}
 
 	# Now move back postgresql.conf with old settings
-	move(
-		"$testroot/master-postgresql.conf.tmp",
-		"$test_master_datadir/postgresql.conf");
+	move("$tmp_check/master-postgresql.conf.tmp",
+		 "$test_master_datadir/postgresql.conf");
 
 	# Plug-in rewound node to the now-promoted standby node
 	append_to_file(