fix: test: Improve key collision detection in ksr system test

MR !10238 added key collision detection in the ksr system test but it was flawed because for every "collide" in the output we also log
"Generating an new key" and for each "Generating" we add the counter by one, nullifying the subtract by one.

Fix by splitting the output on ':' rather than on the default whitespace. Also make the substring matching more strict.

Closes #5229 (again)

Merge branch '5229-ksr-key-collision' into 'main'

See merge request isc-projects/bind9!10775