sum up all occurrences when searching in multiple files

For example, calling file_contains_line_n_times on 'kea-legal*.txt' now shows the total amount of occurences from all matched files. Previously, it would fail with:

ValueError: invalid literal for int() with base 10: '/usr/local/var/lib/kea/kea-legal.20221018.txt:1\n/usr/local/var/lib/kea/kea-legal.20221019.txt:2'

https://jenkins.aws.isc.org/job/kea-dev/job/tarball-system-tests/700/testReport/junit/tests.dhcp.hooks/test_legal_log/run_tests___ubuntu_20_04_v4___test_v4_legal_log_assigned_address_via_relay_one_address/