zdtm.py: cache test messages

Currently, we rely on the fact that nobody else is using random and
parent and child processes will get the same random values.

Now, this test fails on the fedora rawhide. There will be nothing wrong
if we will generate test messages before forking the test process.
Signed-off-by: Andrei Vagin <avagin@gmail.com>

test/zdtm.py

@@ -584,9 +584,14 @@ class inhfd_test:
 		self.__files = None
 		self.__peer_file_names = []
 		self.__dump_opts = []
+		self.__messages = {}
 
 	def __get_message(self, i):
-		return b"".join([random.choice(string.ascii_letters).encode() for _ in range(10)]) + b"%06d" % i
+		m = self.__messages.get(i, None)
+		if not m:
+			m = b"".join([random.choice(string.ascii_letters).encode() for _ in range(10)]) + b"%06d" % i
+		self.__messages[i] = m
+		return m
 
 	def start(self):
 		self.__files = self.__fdtyp.create_fds()