[th/print-results-multiple] print_results: support printing multiple results

[thom311]

Since "config.yaml" can contain multiple tests and "tft.py" can write multiple result files, so also "print_results.py" should handle multiple files.

That way,

./tft.py -o ft-logs/result- config.yaml
./print_results.py ft-logs/result-*.json

works to check all result files, regardless how many tests are defined in config.yaml.

[thom311]

this is done because in https://gitlab.cee.redhat.com/ccit/jenkins-csb-customers/nst-casc-net-hw-ci/-/blob/5e59d37e2773b6d28ee180490aadf900eb2cbaed/scripts/ovn-traffic-flow/traffic-flow-run-test.jenkinsfile#L59 we do:

./print_results.py $(ls ft-logs/*-RESULTS)

previously, that only works because the used config.yaml happens to only contain one test. This line should always work correctly, regardless how many test results are created by tft.py.