Improve loading of test configurations

[jmf-tls]

Currently the test configurations are stored in a properties file which is parsed each time a test loads those configurations; the configs are then matched with the vendors requested by a test.
Instead of loading and parsing the file each time, we could have those configs memoized (see #297 (comment)).

Better yet, we could keep the properties file and load the configs from there just to check if the loading and processing of configs works, but for the majority of the tests we could have the configs in an enum. The tests would then fetch the configs they need by providing the respective enum values.