runtests.pl: support multiline <postcheck> commands

tests/runtests.pl

@@ -3793,7 +3793,7 @@ sub singletest {
     # run the postcheck command
     my @postcheck= getpart("client", "postcheck");
     if(@postcheck) {
-        $cmd = $postcheck[0];
+        $cmd = join("", @postcheck);
         chomp $cmd;
         subVariables \$cmd;
         if($cmd) {