zdtm: print 5 lines before an error

It's very hard to understand an error without a context
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm.py

@@ -1552,13 +1552,19 @@ def print_error(line):
 def grep_errors(fname):
 	first = True
 	print_next = False
+	before = []
 	for l in open(fname):
+		before.append(l)
+		if len(before) > 5:
+			before.pop(0)
 		if "Error" in l:
 			if first:
 				print_fname(fname, 'log')
 				print_sep("grep Error", "-", 60)
 				first = False
-			print_next = print_error(l)
+			for i in before:
+				print_next = print_error(i)
+			before = []
 		else:
 			if print_next:
 				print_next = print_error(l)