Add comment to btest script.

tests/ssh_tests/placeholder.bro

@@ -1,3 +1,6 @@
 # @TEST-EXEC: bro -C -r $TRACES/known_host_test.pcap %INPUT
 # @TEST-EXEC: bro-cut id.orig_h id.resp_h < conn.log > conn.tmp && mv conn.tmp conn.log
 # @TEST-EXEC: btest-diff conn.log
+
+# Since this script relies on DNS responses, basically all we can really
+# do is make sure the script loads successfully and Zeek runs.