Fix context pollution in IterationNode.around()

[leonard84]

Prior to this commit, any exception thrown in IterationNode.around(), i.e. in an iteration interceptor,
would pollute the feature context, affecting the outcome of subsequent iterations and the feature itself.

fixes #1441

[codecov]

Codecov Report

Merging #1446 (3ef018d) into master (e9d5ece) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 3ef018d differs from pull request most recent head 4dd436c. Consider uploading reports for the commit 4dd436c to get more accurate results

@@             Coverage Diff              @@
##             master    #1446      +/-   ##
============================================
+ Coverage     79.52%   79.53%   +0.01%     
- Complexity     4006     4007       +1     
============================================
  Files           404      404              
  Lines         12547    12550       +3     
  Branches       1641     1641              
============================================
+ Hits           9978     9982       +4     
+ Misses         1974     1973       -1     
  Partials        595      595              

Impacted Files	Coverage Δ
...java/org/spockframework/runtime/IterationNode.java	100.00% <100.00%> (ø)
...ain/java/org/spockframework/runtime/SpockNode.java	78.72% <0.00%> (+2.12%)	⬆️

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