Improve roundtrip tests performance

[mmsmits]

Question from @GinoCanessa

A lot of the 'long' tests are reading and writing contents to disk for all intermediate stages - are those artifacts necessary somewhere, or could we remove them?

For example, skipping the intermediate file IO on my machine takes the execution of FullRoundtripOfAllExamplesJsonNewSerializer from 4.1 minutes down to 25.3 seconds.

It would be very nice to make this faster, that would improve the duration of our build pipeline greatly.

[ewoutkramer]

Note: even the original FullRoundTripOfallExamplesJsonNewSerializer only took 40 seconds on my machine, so I think Gino has an anti-virus messing with his temp output directory! Even so, I'll make all of this in memory (per file ;-)) instead of using file storage as an intermediate.