Drop the rndc "-t" option from shudown system test

Michal Nowak requested to merge mnowak/shutdown-test-drop-t-option into bind-9.18

BIND 9.18 rndc does not know about the "-t" option, that results in an silent "invalid argument" error.

----------------------------- Captured stderr call -----------------------------
lt-rndc: invalid argument -t
Usage: lt-rndc [-b address] [-c config] [-s server] [-p port]
	[-k key-file ] [-y key] [-r] [-V] [-4 | -6] command

The "-t" option originally came with 184eddb5 (a backport from the main branch).

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