syslog: accept standard rsyslogd rate-limit message in test_syslog_rate_limit

The test expected only SONiC's patched rsyslogd message:
  'begin to drop messages due to rate-limiting'

Standard rsyslogd (used on some platforms) emits a different message:
  'N messages lost due to rate-limiting (M allowed within K seconds)'

Both messages indicate that rate limiting is working correctly. Use
regex alternation to accept either form, making the test work across
different rsyslogd builds.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Bing Wang <bingwang@microsoft.com>

Author: bingwang-ms

tests/syslog/test_syslog_rate_limit.py

@@ -21,8 +21,11 @@
 LOCAL_LOG_GENERATOR_FILE = os.path.join(BASE_DIR, 'log_generator.py')
 REMOTE_LOG_GENERATOR_FILE = os.path.join('/tmp', 'log_generator.py')
 DOCKER_LOG_GENERATOR_FILE = '/log_generator.py'
-# rsyslogd prints this log when rate-limiting reached
-LOG_EXPECT_SYSLOG_RATE_LIMIT_REACHED = '.*rate-limit-test>: begin to drop messages due to rate-limiting.*'
+# rsyslogd emits one of two messages depending on version when rate-limiting kicks in:
+#   - "begin to drop messages due to rate-limiting"  (logged when drops start)
+#   - "N messages lost due to rate-limiting (M allowed within K seconds)"  (logged as summary)
+# Accept either form so the test works across different rsyslogd versions.
+LOG_EXPECT_SYSLOG_RATE_LIMIT_REACHED = r'.*(?:begin to drop messages|messages lost) due to rate-limiting.*'
 # Log pattern for tests/syslog/log_generator.py
 LOG_EXPECT_LAST_MESSAGE = '.*{}rate-limit-test: This is a test log:.*'