Fix hanging test262 bug by setting the length on String objects

[jasonwilliams]

• Set a length on new String objects
• Add a test

This Pull Request fixes/closes #720.

[codecov]

Codecov Report

Merging #728 into master will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #728      +/-   ##
==========================================
- Coverage   71.80%   71.75%   -0.06%     
==========================================
  Files         199      200       +1     
  Lines       13944    13983      +39     
==========================================
+ Hits        10012    10033      +21     
- Misses       3932     3950      +18     

Impacted Files	Coverage Δ
boa/src/exec/tests.rs	100.00% <100.00%> (ø)
boa/src/value/mod.rs	69.55% <100.00%> (ø)
boa/src/syntax/parser/error.rs	24.19% <0.00%> (-6.46%)	⬇️
boa/src/syntax/parser/cursor/mod.rs	77.55% <0.00%> (-6.13%)	⬇️
boa/src/exec/iteration/mod.rs	58.20% <0.00%> (-2.36%)	⬇️
boa/src/syntax/ast/punctuator.rs	6.25% <0.00%> (-1.05%)	⬇️
boa/src/syntax/parser/statement/mod.rs	56.42% <0.00%> (-0.72%)	⬇️
boa/src/exec/iteration/tests.rs	100.00% <0.00%> (ø)
...oa/src/syntax/parser/statement/labelled_stm/mod.rs	86.66% <0.00%> (ø)
boa/src/syntax/ast/node/iteration.rs	61.79% <0.00%> (+1.30%)	⬆️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 29560a5...5523e48. Read the comment docs.