fix: correct dut variable name in startup tsa pretest (sonic-net#18699)

Description of PR
Fix the dut variable name in test_disable_startup_tsa_tsb_service() in test_pretest.py added in sonic-net#17247

Summary:
Fixes # (issue)

Type of change
 Bug fix
 Testbed and Framework(new/improvement)
 New Test case
 Skipped for non-supported platforms
 Test case improvement
Back port request
 202205
 202305
 202311
 202405
 202411
 202505
Approach
What is the motivation for this PR?
When using multithreading parallel run, we cannot refer to the duthost variable, we need to use dut instead.

Author: cyw233

tests/test_pretest.py

@@ -461,7 +461,7 @@ def disable_startup_tsa_tsb(dut):
                 dut.shell("sudo mv {} {}".format(startup_tsa_tsb_file_path, backup_tsa_tsb_file_path))
                 output = dut.shell("TSB", module_ignore_errors=True)
                 pytest_assert(not output['rc'], "Failed TSB")
-                duthost.shell("sudo config save -y")
+                dut.shell("sudo config save -y")
         else:
             logger.info("{} file does not exist in the specified path on dut {}".
                         format(startup_tsa_tsb_file_path, dut.hostname))