Added unit tests for boa_ast::Keyword

[Razican]

We have a very low code coverage in our tests, and, while we have a great end-to-end testing framework setup (test262), this doesn't run unit tests for our Rust implementation.

This is the first of a series of PRs that will try to increase our code coverage so that it reaches reasonable levels.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	94,601	94,601	0
Passed	73,293	73,293	0
Ignored	17,540	17,540	0
Failed	3,768	3,768	0
Panics	0	0	0
Conformance	77.48%	77.48%	0.00%

[codecov]

Codecov Report

Merging #2883 (b2b7477) into main (c738b50) will increase coverage by 0.18%.
The diff coverage is n/a.

❗ Current head b2b7477 differs from pull request most recent head 9f9d5f8. Consider uploading reports for the commit 9f9d5f8 to get more accurate results

@@            Coverage Diff             @@
##             main    #2883      +/-   ##
==========================================
+ Coverage   51.24%   51.42%   +0.18%     
==========================================
  Files         427      427              
  Lines       42522    42520       -2     
==========================================
+ Hits        21790    21868      +78     
+ Misses      20732    20652      -80     

Impacted Files	Coverage Δ
boa_ast/src/keyword/mod.rs	100.00% <ø> (ø)

[HalidOdat]

Looks good to me! :)

Just a small question