Performance enhancement: only download and parse license & exception JSON files once

[pmonks]

I've noticed that some JSON files are downloaded (and presumably also parsed) repeatedly during the unit tests, which seems to suggest that a performance enhancement may be possible whereby those files are only downloaded and parsed into memory once, then reused. It seems to mostly (only?) happen for the JSON files for individual licenses and exceptions (e.g. https://spdx.org/licenses/Apache-2.0.json), and in issue #193 @goneall confirmed that that was his understanding too.