tc-dtka test does not say that it will sleep for 8 minutes

[250MHz]

The tc-dtka test sleeps for 8 minutes without telling the user that it will be sleeping. The comment above incorrectly says 7 minutes.

ION-DTN/tests/tc-dtka/runtest

Lines 216 to 217 in bf89726

	# Sleep another 7 minutes, note that second re-keying cycle should be done.
	sleep 480

This gives the impression that something broke in the test.
In contrast, when the test sleeps for 6 minutes, it does say so:

ION-DTN/tests/tc-dtka/runtest

Lines 185 to 189 in bf89726

	echo "Finished loading initial keys, now sleeping 6 minutes."
	# Sleep 360 seconds, note that first re-keying cycle should have completed.
	sleep 360