Add comments to autotests

QtScript module is extensively tested by automatic tests. Not only API is tested, but also ECMA Script compliance (look also at QTBUG-16606), behavior and many different "edge cases". On one hand this allow us to quickly find regressions and ensure really good stability. On other hand many tests "enforce" behavior even if we are not sure if it is sensible or important.

For example tst_QScriptValueIterator, it enforce iterating order even if ECMA Script describe it as "undefined". Nobody should relay on this behavior, even if the test may suggest something different.

I would suggest to:
1. Add, inside test code, comments that describe purpose of the test

• what is the goal of the test, what behavior it actually tests
• why it was written (bug report, crash, ECMA standard...)
• impact of breaking it (none, corner case, ECMA compliance...)
  2. Split tests into really small chunks
  3. Rewrite some tests (if possible) to not define undefined behaviors.