test: only assert on first lines of TLS trace

The TLS trace data is best-effort, and enough can be dropped from pipe
buffers that only the start of the trace is detected. Only assert on the
first line of the trace, it should not get dropped, and it's enough to
check that trace was enabled via CLI.

PR-URL: https://github.com/nodejs/node/pull/28043
Fixes: https://github.com/nodejs/node/issues/27636


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>