Improve e2e test performance

Using Gradle, end-to-end NSHM tests take a very long time to run. Locally it looks like all cores are swamped and it's not clear what's going on, but something about doing the multi-threaded hazard calculations within the test framework is not working.

Generating results outside the test framework works as expected and so in !754 (merged) the gradle tests have been restructured to generate and write results so that the test just compares values in JSON files.

I think it would be better to have both the local and gradle tests compare results written to file and some reorganization could be done to make things in the code a little clearer (distinction between expected and actual).