Add perl2host call missing from a new test file.

Oversight in today's commit c6b92041d3.
Per buildfarm member jacana.

Discussion: http://postgr.es/m/20200404223212.GC3442685@rfd.leadboat.com

src/test/recovery/t/018_wal_optimize.pl

@@ -124,6 +124,7 @@ wal_skip_threshold = 0
 		$copy_file, qq(20000,30000
 20001,30001
 20002,30002));
+	$copy_file = TestLib::perl2host($copy_file);
 
 	# Test truncation with inserted tuples using both INSERT and COPY.  Tuples
 	# inserted after the truncation should be seen.