Fix a bunch of minor issues and repetitions

[na--]

I started commenting on a bunch of minor issues in #1428, but then realized that patching them would be simpler than explaining them, even without the back-and-forth comments... 😅

@imiric, if you don't find any issues with this, feel free to merge it in #1428 and then merge your PR.

[na--]

hmm wait, I think I found another minor issue, investigating...

[na--]

Previously, running this code with k6 run --env aaa=bbb --include-system-env-vars=false script.js:

export default function () {
  console.log(`Start VU ${__VU} ITER ${__ITER}, ENV ` + JSON.stringify(__ENV));
  __ENV.asdfg = 123123;
}

export let options = {
  execution: {
    scenario1: {
      type: "per-vu-iterations",
      vus: 1,
      iterations: 1,
      maxDuration: "3s",
      env: {
        "111": "111",
      },
      gracefulStop: 0,
    },
    scenario2: {
      type: "per-vu-iterations",
      vus: 1,
      iterations: 1,
      startTime: "3s",
      maxDuration: "10s",
      env: {
        "222": "222",
      },
    },
  },
};

would have outputted something like this:

INFO[0000] Start VU 1 ITER 0, ENV {"aaa":"bbb","111":"111"} 
INFO[0003] Start VU 1 ITER 1, ENV {"aaa":"bbb","111":"111","asdfg":"123123","222":"222"} 

after a345f5c, it will output

INFO[0002] Start VU 1 ITER 0, ENV {"aaa":"bbb","111":"111"} 
INFO[0005] Start VU 1 ITER 1, ENV {"aaa":"bbb","222":"222"} 

Which is much saner, even if it might be considered a slight breaking change. Though probably not, I personally consider relying on __ENV persistence between VU reuses relying on undefined behavior... Also, VU reuses happened only in stages or iterations, so this will actually mostly continue to work, since in the new paradigm, these will be single executors, so __ENV persistence will continue to work like before for these cases.

Anyway, this probably deserves its own minor test, but I don't have time to write it now, so @imiric, feel free to write one (or extend one of your existing ones) if you want.

[imiric]

Thanks for the env fix, I didn't consider VU reuse... 🤦 I'll add a test for it before merging #1428.

And cheers for the group tag handling in a single place and where it belongs now, though having to set it in tests is a bit unfortunate, but not a big deal.