zdtm: don't print one line twice

(00.566486)pie: 1: seccomp: mode 0 on tid 1
(00.566601)pie: 1: Error (criu/pie/restorer.c:460): seccomp: Unexpected tid ->
(00.566601)pie: 1: 1 != 1
(00.566601)pie: 1: 1 != 1
(00.566639)pie: 1: Error (criu/pie/restorer.c:1817): Restorer fail 1
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm.py

@@ -1728,6 +1728,7 @@ def grep_errors(fname):
 		else:
 			if print_next:
 				print_next = print_error(l)
+				before = []
 	if not first:
 		print_sep("ERROR OVER", "-", 60)