Cleanup and added test for String.prototype.concat

[HalidOdat]

It changes the following:

• Uncomented one of the concat test case (the other one is fixed by Fix all Value operations and add unsigned shift right #520)
• Added a test case for a generic concat method (as described in spec)
• Removed deprecated comment.

[codecov]

Codecov Report

Merging #538 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
+ Coverage   68.31%   68.34%   +0.02%     
==========================================
  Files         171      171              
  Lines       10486    10494       +8     
==========================================
+ Hits         7164     7172       +8     
  Misses       3322     3322              

Impacted Files	Coverage Δ
boa/src/builtins/string/mod.rs	37.28% <ø> (ø)
boa/src/builtins/string/tests.rs	100.00% <100.00%> (ø)

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[github-actions]

Benchmark for 1c7c066

Click to view benchmark

Test	PR Benchmark	Master Benchmark	%
Arithmetic operations (Execution)	588.5±20.54ns	590.1±19.51ns	-0.27%
Array access (Execution)	13.8±0.60µs	14.5±0.56µs	-4.83%
Array creation (Execution)	3.9±0.12ms	4.1±0.20ms	-4.88%
Array pop (Execution)	1498.4±91.79µs	1539.2±54.14µs	-2.65%
Boolean Object Access (Execution)	5.4±0.17µs	5.5±0.27µs	-1.82%
Create Realm	138.9±16.03µs	145.1±8.42µs	-4.27%
Dynamic Object Property Access (Execution)	7.4±0.29µs	7.4±0.35µs	0.00%
Expression (Lexer)	2.2±0.08µs	2.2±0.07µs	0.00%
Expression (Parser)	5.3±0.15µs	5.4±0.16µs	-1.85%
Fibonacci (Execution)	981.8±48.40µs	1044.4±42.78µs	-5.99%
For loop (Execution)	22.8±1.20µs	24.3±1.76µs	-6.17%
For loop (Lexer)	5.6±0.17µs	5.7±0.38µs	-1.75%
For loop (Parser)	14.1±0.62µs	14.2±0.37µs	-0.70%
Hello World (Lexer)	1027.8±59.63ns	1034.3±54.86ns	-0.63%
Hello World (Parser)	2.3±0.12µs	2.3±0.12µs	0.00%
Long file (Parser)	6.4±0.28ms	6.4±0.12ms	0.00%
Number Object Access (Execution)	4.4±0.24µs	4.3±0.12µs	+2.33%
Object Creation (Execution)	6.3±0.53µs	6.5±0.21µs	-3.08%
RegExp (Execution)	76.7±3.76µs	76.3±2.64µs	+0.52%
RegExp Literal (Execution)	81.8±7.02µs	80.8±3.89µs	+1.24%
RegExp Literal Creation (Execution)	76.2±3.12µs	76.2±3.52µs	0.00%
Static Object Property Access (Execution)	6.7±0.26µs	6.6±0.20µs	+1.52%
String Object Access (Execution)	9.3±0.28µs	9.3±0.37µs	0.00%
String comparison (Execution)	7.2±0.24µs	7.3±0.36µs	-1.37%
String concatenation (Execution)	6.1±0.30µs	5.9±0.19µs	+3.39%
String copy (Execution)	4.8±0.41µs	4.8±0.17µs	0.00%
Symbols (Execution)	4.1±0.24µs	4.3±0.15µs	-4.65%

[github-actions]

Benchmark for ac31798

Click to view benchmark

Test	PR Benchmark	Master Benchmark	%
Arithmetic operations (Execution)	412.8±47.84ns	414.5±48.18ns	-0.41%
Array access (Execution)	15.0±1.66µs	14.7±1.29µs	+2.04%
Array creation (Execution)	3.5±0.31ms	3.8±0.31ms	-7.89%
Array pop (Execution)	1291.0±113.48µs	1335.5±129.46µs	-3.33%
Boolean Object Access (Execution)	5.2±0.57µs	5.2±0.49µs	0.00%
Create Realm	152.7±16.92µs	163.9±13.41µs	-6.83%
Dynamic Object Property Access (Execution)	7.7±0.81µs	7.4±0.78µs	+4.05%
Expression (Lexer)	2.2±0.26µs	2.1±0.21µs	+4.76%
Expression (Parser)	4.8±0.48µs	5.4±0.61µs	-11.11%
Fibonacci (Execution)	1032.9±89.05µs	1096.9±94.92µs	-5.83%
For loop (Execution)	22.6±2.46µs	24.4±2.81µs	-7.38%
For loop (Lexer)	5.0±0.30µs	5.4±0.43µs	-7.41%
For loop (Parser)	14.1±1.76µs	13.7±1.56µs	+2.92%
Hello World (Lexer)	944.6±103.71ns	930.2±90.95ns	+1.55%
Hello World (Parser)	2.1±0.21µs	2.3±0.25µs	-8.70%
Long file (Parser)	6.0±0.35ms	6.2±0.48ms	-3.23%
Number Object Access (Execution)	4.1±0.35µs	4.1±0.38µs	0.00%
Object Creation (Execution)	6.3±0.70µs	6.6±0.59µs	-4.55%
RegExp (Execution)	80.5±12.58µs	77.7±7.48µs	+3.60%
RegExp Literal (Execution)	82.2±8.40µs	83.2±7.21µs	-1.20%
RegExp Literal Creation (Execution)	75.8±18.69µs	80.7±9.91µs	-6.07%
Static Object Property Access (Execution)	6.3±0.37µs	7.1±0.78µs	-11.27%
String Object Access (Execution)	8.7±1.01µs	8.7±0.91µs	0.00%
String comparison (Execution)	7.3±0.91µs	7.4±0.91µs	-1.35%
String concatenation (Execution)	5.8±0.47µs	6.0±0.68µs	-3.33%
String copy (Execution)	4.7±0.47µs	4.8±0.64µs	-2.08%
Symbols (Execution)	4.2±0.47µs	4.2±0.36µs	0.00%