Improve e2e test coverage around environment storage

[mxsdev]

We have had a few bugs around environment variables; namely, the environment could "overflow" by becoming too large, and it could also break if any cell's output had null characters.

It would be nice to have some e2e tests around the environment storage to prevent regressions in these areas.

One idea is to take a chaos monkey kind of approach, doing stress tests by trying to pump lots of random bytes into the environment.