Fix Unicode character escape sequence parsing

[tofpie]

This Pull Request fixes #827.

It changes the following:

• Parsing of Unicode escape sequences
• Lexer tests

[codecov]

Codecov Report

Merging #959 (1f6119b) into master (98e11d7) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #959      +/-   ##
==========================================
+ Coverage   59.29%   59.32%   +0.02%     
==========================================
  Files         166      166              
  Lines       10689    10687       -2     
==========================================
+ Hits         6338     6340       +2     
+ Misses       4351     4347       -4     

Impacted Files	Coverage Δ
boa/src/syntax/lexer/string.rs	38.09% <ø> (-1.44%)	⬇️
boa/src/syntax/lexer/mod.rs	68.46% <0.00%> (+3.60%)	⬆️

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[Razican]

Thanks for the fix! Looks very good to me, let's merge :)

[Razican]

For posterity's sake, here we have the results of the test262 test suite. Surprisingly, there are no tests for this (or there is something we are missing):

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,460	78,460	0
Passed	18,964	18,964	0
Ignored	15,559	15,559	0
Failed	43,937	43,937	0
Panics	1,127	1,127	0
Conformance	24.17	24.17	0.00%