k6/execution:Don't panic on accessing test.options

[mstoykov]

What?

Fixes a panic when accessing k6/execution.test.options in the init context.

Why?

Not to panic but give an exception.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.30%. Comparing base (b866c10) to head (9c2d7cf).

❗ Current head 9c2d7cf differs from pull request most recent head d357f49. Consider uploading reports for the commit d357f49 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3708      +/-   ##
==========================================
+ Coverage   73.28%   73.30%   +0.02%     
==========================================
  Files         278      278              
  Lines       20389    20392       +3     
==========================================
+ Hits        14942    14949       +7     
+ Misses       4497     4493       -4     
  Partials      950      950              

Flag	Coverage Δ
ubuntu	73.23% <100.00%> (+0.01%)	⬆️
windows	73.16% <100.00%> (+0.02%)	⬆️

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[olegbespalov]

LGTM. Maybe we could use common.NewInitContextError helper.