Warn on access to the exported script options instead of test.options in k6/execution

[na--]

From https://github.com/grafana/k6/pull/2571/files#r921222735

There are various issues when scripts try to access the exported script options from the script. #2571 fixes one bug/undefined behavior, but there are a few other subtle problems.

I think it would be relatively easy to make this into a DynamicObject that prints a warning if anyone tries to access any of the properties? 🤔

We can warn people (with a sync.Once, to limit spam) to use test.options from k6/execution for read (i.e. Get(), Keys() and Has()) operations. And also warn (with another sync.Once) them that modifying the options (i.e. on Set() and Delete()) during the script runtime is useless and won't actually affect anything during the script execution.

[na--]

This will probably be tricky to implement, since we will have to do it on a key by key basis, not on the whole options object, see the discussion in #2571 (comment) 😞