Fix log_retention test: use correct test run variable (#12884)

Test 7 ('Get the log to rotate.') was calling MakeCurlCommandMulti on
test.tr (the test run from the TestLogRetention constructor) instead of
the newly created tr. This left the new test run with no process
defined, causing the test to fail with 'List came back empty.'

Author: brbzull0

tests/gold_tests/logging/log_retention.test.py

@@ -488,7 +488,7 @@ def get_command_to_rotate_thrice(self):
 tr.StillRunningAfter = test.server
 
 tr = Test.AddTestRun("Get the log to rotate.")
-test.tr.MakeCurlCommandMulti(test.get_command_to_rotate_once(), ts=test.ts)
+tr.MakeCurlCommandMulti(test.get_command_to_rotate_once(), ts=test.ts)
 tr.Processes.Default.ReturnCode = 0
 tr.StillRunningAfter = test.ts
 tr.StillRunningAfter = test.server