Fix attribute configurable of the length property of arguments

[tofpie]

This PR fixes the attribute configurable of the length property of arguments.

[codecov]

Codecov Report

Merging #1071 (d0b42e7) into master (aa03a7d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1071   +/-   ##
=======================================
  Coverage   58.69%   58.69%           
=======================================
  Files         176      176           
  Lines       12389    12389           
=======================================
  Hits         7272     7272           
  Misses       5117     5117           

Impacted Files	Coverage Δ
boa/src/builtins/function/mod.rs	69.64% <100.00%> (ø)

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[Razican]

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,497	78,497	0
Passed	24,795	24,799	+4
Ignored	15,587	15,587	0
Failed	38,115	38,111	-4
Panics	26	26	0
Conformance	31.59	31.59	+0.01%